OBJECTIVES: Capecitabine is an orally administered prodrug of fluorouracil, which is used for the treatment of colorectal and breast cancer patients. Since patients take this drug usually at home a high adherence to the prescribed dosage regimen is a major prerequisite for therapeutic efficacy. METHODS: Adherence to capecitabine chemotherapy was assessed using MEMS(TM)-vials. The closures of the vials are equipped with an integrated micro-electronic circuit registering every opening and closing. With the corresponding software the medication events of the particular patient can be displayed on a computer and evaluated. Percentage of correctly administered doses of medication (i.e. registered openings of the MEMS(TM)-vial at the correct time), and dosage (i.e. opening) intervals were defined as main outcome measures. RESULTS: We assessed the adherence to capecitabine chemotherapy (taken twice daily for 14 days with a 7 days break between the cycles) in 20 patients (14 breast and 6 colorectal cancer) over a mean period of 44.3 treatment days (Range = 14 to 79 days, Median = 44.3 days, SD = 18.9 days). Overall adherence varied from 85.5% to 100% (Mean = 96.7%, Median = 98.9%, SD = 4.3%). The actual vial opening intervals among the studied patients varied from 1.6 hours to 18.9 hours (Mean = 12.0 h, Median = 12.0 h, SD = 1.7 h, including only days with actual twice daily opening). CONCLUSIONS: Although the results show in general a high degree of overall adherence and interval regularity, there were several individuals who exhibited major discrepancies that may potentially endanger therapeutic goals. The adherence data demonstrated here form the basis for a prospective, open, controlled, two-armed observational study investigating the impact of pharmaceutical care as a multidisciplinary patient-oriented service on adherence and other relevant clinical, subjective and economic outcome parameters in a larger patient population.