Adherence to analgesics in cancer patients has scarcely been studied. In this study, the Medication Event Monitoring System (MEMS) and medication diaries were compared with respect to feasibility and adherence measurements. Forty-six outpatients with nociceptive pain caused by cancer were asked to use MEMS for their analgesics and to record their medication usage in a diary for four weeks. Seventy-nine percent of the patients used MEMS for the full four-week period; 70% did so for the diary. The majority of patients were satisfied with both MEMS and diary. Adherence data assessed by MEMS and diary were comparable. Patients used the amount of analgesics adequately (taking adherence: 87%) but took them irregularly (timing adherence: 53%). Subgroup analyses in patients using single and multiple analgesic regimens confirmed the comparable suitability of both methods. MEMS and a medication diary are equally useful for analgesic adherence measurement in cancer patients with pain.