BACKGROUND: Impaired impulse control is often seen in Parkinson's disease (PD) patients using dopamine agonists. METHODS: We performed a therapeutic drug monitoring study of 100 PD patients using ropinirole or pramipexole extended release. Three blood samples were collected on the same day. Serum concentrations were measured, and 24 h area under the curve (AUC) calculated. The validated Questionnaire for Impulsive-Compulsive Disorders in Parkinson's Disease-Rating Scale (QUIP-RS) was used for assessing impulse control. RESULTS: Total ropinirole drug exposure showed weak, but significant correlation to the QUIP-RS score. No correlation between pramipexole serum concentrations and QUIP-RS was found. In ropinirole patients, both agonist dose and total dopaminergic treatment were correlated with QUIP-RS. Duration of ropinirole treatment correlated with impaired impulse control, and duration of dopaminergic treatment of any type correlated with QUIP-RS scores in both ropinirole and pramipexole patients. CONCLUSIONS: Our main finding is that impaired impulse control is correlated to both total drug exposure (AUC) and dopamine agonist dose for ropinirole, but not for pramipexole. These observations indicate that different strategies may be useful for treating PD patients with impaired impulse control: ropinirole dose reduction could be beneficial, whereas pramipexole treatment may have to be stopped.