Objective: To estimate the expression of haptoglobin (Hp)-βchain isoforms in the peritoneal fluid (PF) and plasma (PL) of women with and without endometrios is. Design: Cross-sectional study. Setting: University hospital. Patient(s): Se venty-two patients with endometriosis and 35 controls. Intervention(s): Periton eal fluid and PL samples were subjected to two-dimensional gel electrophoresis, silver stained, digitally captured, and compared by semiquantitative computeriz ed analysis. Main Outcome Measure(s): Expression of Hp βchain isoforms. Result( s): No significant difference was observed in the frequency of expression of the Hp βchain isoforms between the two groups. One βchain isoform (HpβE; molecul ar weight, 38.40 ±0.94 kD; and isoelectric point, 5.63 ±0.17) had significantl y higher expression in both the PF and PL of women with endometriosis than in th e PF and PL of controls. No significant difference was observed in HpβE express ion between women with mild (revised American Fertility Society [rAFS], stage I -II) and severe (rAFS, stage III-IV) endometriosis. In the control group, Hpβ E expression was correlated with the phase of the menstrual cycle. Conclusion(s) : Women with endometriosis have higher PF and PL levels of HpβE. Further invest igation should be aimed at producing antibodies against the HpβE-specific epit opes to determine whether its measurement may improve the clinical diagnosis of endometriosis. Objective: To estimate the expression of haptoglobin (Hp) -βchain isoforms in the peritoneal fluid (PF) and plasma (PL) of women with and without endometrios is. Design: Cross-sectional study. Setting: University hospital. Patient (s) : Two patients with endometriosis and 35 controls. Intervention (s): Periton eal fluid and PL samples were subjected to two-dimensional gel electrophoresis, silver stained, digitally captured, and compared by semiquantitative computeriz ed analysis. Main Outcome Measure ( s): No significant difference was observed in the frequency of expression of the Hpβchain isoforms between the two groups. One βchain isoform (HpβE; molecul ar weight, 38.40 ± 0.94 kD; and isoelectric point, 5.63 ± 0.17) had significantl y higher expression in both the PF and PL of women with endometriosis than in th PF and PL of controls. No significant difference was observed in HpβE express ion between women with mild (revised American Fertility Society [rAFS], stage I-II) and severe (rAFS, stage III-IV) endometriosis. In the control group, Hpβ E expression was correlated with the phase of the menstrual cycle. Further invest ought should aimed at producing antibodies against the HpβE-specific epitopes to determine whether its measurement may improve the clinical diagnosis of endometriosis.