The objective of this study was to assess different methods of measuring therapy adherence in Parkinson’s disease (PD). In a single centre observational study, 112 patients with idiopathic PD were randomised to a crossover trial of active monitoring (n = 69, simple tablet count and electronic monitoring), or to no monitoring (n = 43, control group). All patients completed a self report and visual analogue scale (VAS) indicating therapy intake. In the active monitoring group, 56 (81% of cases) used ≥ 80% of their medication, and 13 (19% of cases) used < 80% , based on electronic monitoring. Median adherence for self report was 100% (interquartile range (IQR) 100 to 100) and for VAS was 100% (IQR 95 to 100), in both active and control groups. Patients taking ≥ 80% of prescribed medication had a median total adherence of 98% (IQR 93 to 101 ) by electronic monitoring, which was similar to that from other methods: self report 100% , IQR 100 to 100; VAS 100% , IQR 95 to 100; simple tablet count 98% , IQR 89 to 100. Median total adherence in patients taking < 80% of medication was significantly lower by electronic monitoring (69% , IQR 44 to 74) than by other methods: self report 100% , IQR 100 to 100; VAS 100% , IQR 95 to 100; and simple tablet count 90% , IQR 78 to 100 (all p < 0.0001). Sensitivities of self report (10% ), VAS (17% ), and simple tablet count (50% ) were all low for detecting suboptimal medicine intake. Self report, VAS, and simple tablet counts are insensitive as predictors of suboptimal medicine usage in PD. How patients take their medicines influences interpretation of the therapy response and consequent management decisions, with implications for clinical trial analysis and clinical practice. The objective of this study was to assess different methods of measuring therapy adherence in Parkinson’s disease (PD). In a single center observational study, 112 patients with idiopathic PD were randomized to a crossover trial of active monitoring (n = 69, simple tablet count All patients completed a self report and visual analogue scale (VAS) indicating therapy intake. In the active monitoring group, 56 (81% of cases) used ≥ Median adherence for self report was 100% (interquartile range (IQR) 100 to 100) and for VAS was 100% (IQR 95 of 100), in both active and control groups. Patients taking ≥ 80% of prescribed medications had a median total adherence of 98% (IQR 93 to 101) by electronic monitoring, which was similar to that from other methods: self report 100 %, IQR 100 to 100; VAS 100%, IQR 95 to 100; simple tab let count 98%, IQR 89 to 100. Median total adherence in patients taking <80% of the doses significantly reduced by electronic monitoring (69%, IQR 44 to 74) than by other methods: self report 100%, IQR 100 to 100 ; Sensitivities of self report (10%), VAS (17%), and simple tablet count (50%); VAS 100%, IQR 95 to 100; ) were all low for detecting suboptimal medicine intake. Self report, VAS, and simple tablet counts are insensitive as predictors of suboptimal medicine usage in PD. How patients take their Medicine influences interpretation of the therapy response and consequent management decisions, with implications for clinical trial analysis and clinical practice.