Purpose: To study the electroretinographic signals originating in the long-wavelength-sensitive (L) and middle-wavelengthsensitive (M) cone pathways by means of large-field and multifocal cone type-specific electroretinograms (ERGs) in a patient with progressive cone dystrophy. Methods: A 65-year-old male patient with colour vision disturbances (age at onset 10 years), loss of visual acuity (14 years), and central visual field defects (40 years) was investigated. Large-field flicker-ERG responses to stimuli that exclusively modulated the L-cones or the M-cones, or the two simultaneously (both in-phase and in counter-phase), were measured. Short-wavelength-sensitive(S) cones were not modulated. Multifocal ERGs (mfERGs) were also recorded, with a pattern-reversing display that modulated only the L-or the M-cones at equal cone contrasts and average quantal catches. Genetic analysis of L-and M-pigment genes was performed on genomic DNA isolated from peripheral venous blood. Results: The patient showed a normal rod-drivenERG but reduced cone-driven electroretinographic amplitudes with normal implicit times in the International Society for Clinical Electrophysiology of Vision (ISCEV) standard ERG. The large-field flicker-ERG responses to pure L-cone modulation were significantly above noise level but were substantially reduced in comparison with both normal trichromatic subjects and (otherwise normal) deuteranopes. The L-cone driven electroretinographic implicit times and phases were within normal limits. The M-cone driven electroretinographic responses were not detectable. A model fit of all the L-and M-cone driven flicker-ERG data revealed that the responses were exclusively driven by the L-cones. Consistently, the cone type-specific mfERGs showed severely reduced but detectable responses to L-cone-isolating stimuli. The M cone driven multifocal-ERG responses were undistinguishable from noise. The L-and M-pigment gene array consisted of only a single L-pigment gene. The complete coding sequence of this gene was determined and showed no abnormality. Conclusions: This patient exhibits a coincidence of progressive cone dystrophy and deuteranopia. The molecular genetic data of the L/M-pigment gene array is consistent with the deutan phenotype. It cannot be excluded that the rearrangement of the X-chromosome pigment gene array is responsible for the cone dystrophy in this patient. It is, however, suggested that the dichromacy and the cone dystrophy have different and independent genetic origins. Purpose: To study the electroretinographic signals originating in the long-wavelength-sensitive (L) and middle-wavelength sensitive (M) cone pathways by means of large-field and multifocal cone type-specific electroretinograms (ERGs) in a patient with progressive cone dystrophy . Methods: A 65-year-old male patient with color vision disturbances (age at onset 10 years), loss of visual acuity (14 years), and central visual field defects (40 years) was investigated. Large-field flicker-ERG responses to stimuli that exclusively modulated the L-cones or the M-cones, or the two simultaneously (both in-phase and in counter-phase), were measured. Short-wavelength-sensitive (S) cones were not modulated. Multifocal ERGs (mfERGs) were also recorded with a pattern-reversing display that modulated only the L-or the M-cones at equal cone contrasts and average quantal catches. Genetic analysis of L-and M-pigment genes was performed on genomic DNA isolated from peripheral venous blood. Results: The pa tient showed a normal rod-drivenERG but reduced cone-driven electroretinographic amplitudes with normal implicit times in the International Society for Clinical Electrophysiology of Vision (ISCEV) standard ERG. The large-field flicker-ERG responses to pure L-cone modulation were significantly above noise level but were substantially reduced in comparison with both normal trichromatic subjects and (otherwise normal) deuteranopes. The L-cone driven electroretinographic implicit times and phases were within normal limits. A M-cone driven electroretinographic responses were not detectable. A model fit of all the L-and M-cone driven flicker-ERG data revealed that the responses were exclusively driven by the L-cones. Consistently, the cone type-specific mfERGs showed severely reduced but detectable responses to L-cone-isolating stimuli. The M cone driven multifocal-ERG responses were indiscerishable from noise. The L-and M-pigment gene array consisted of only a single L-pigment gene.complete coding sequence of this gene was determined and showed no abnormality. Conclusions: This patient exhibits a coincidence of progressive cone dystrophy and deuteranopia. The molecular genetic data of the L / M-pigment gene array is consistent with the deutan phenotype. It can not be excluded that the rearrangement of the X-chromosome pigment gene array is responsible for the cone dystrophy in this patient. It is however, suggested that the dichromacy and the cone dystrophy have different and independent genetic origins.