The evaluation of the efficacy of medication is confounded when patients do not adhere to prescribed regimens. Overdosing, underdosing, and erratic dosing intervals can diminish drug action or cause adverse effects. Using a new method with epilepsy as a model, we assessed compliance with long-term medications among newly treated and long-term patients. Medication Event Monitor Systems (Aprex Corporation, Fremont, Calif) are standard pill bottles with microprocessors in the cap to record every bottle opening as a presumptive dose. Compliance rates averaged 76% during 3428 days observed: 87% of the once daily, 81% of the twice daily, 77% of the three times a day, and 39% of the four times a day dosages were taken as prescribed. Coefficients of variation of drug serum concentrations had no significant relationship to compliance rates. Pill counts overestimated compliance increasingly as compliance with the prescribed regimen declined. Neither drug serum concentrations nor pill counts would have identified the frequency of missed doses that were revealed with continuous dose observations.