Patients take less medication than prescribed in many disease areas but evidence for suboptimal therapy adherence in Parkinson's disease (PD) is limited. A single-center observational study of antiparkinsonian medication was undertaken using electronic monitoring (MEMS; Aardex, Zug, Switzerland) over 3 months. Of 68 patients approached, 6 declined and 8 dropped out, leaving 54 patients (taking 117 preparations) with available data. Poorer compliance was associated significantly with younger age, with taking more antiparkinsonian tablets per day, with higher depression scores, and with poorer quality of life. Of the 54 evaluable patients, 11 (20%) had average total compliance of under 80% (underusers) and 43 (80%) had average total compliance of over 80% (satisfactory adherence). Underusers had median total compliance of 65% (interquartile range, 37-74) versus 98% (interquartile range, 93-102) in the satisfactory adherence group. Timing compliance (number of doses taken in the correct time interval) was poor in both underusers (median, 11%; interquartile range, 2-20) and those with satisfactory adherence (median, 25%; interquartile range, 11-73). In conclusion, poorer compliance is associated with younger age, depression, and more tablets per day, and one-fifth of PD patients underuse medication. Consideration of drug therapy adherence has implications in the management of PD.