Antibiotics are frequently prescribed in everyday practice for the management of acute microbial infections. The present study was designed to assess the relationship between the prescriber's instructions and the patient's adherence to a prescribed schedule of twice-daily doses of antibiotic for at least 5 days to treat an infectious disease. The trial was conducted by ten practicing physicians on ambulatory patients. Compliance with the antibiotic regimen was evaluated using a microelectronic device, the Medication Event Monitoring System (MEMS). Seventy patients were prescribed an antibiotic in twice-daily doses for 5 to 14 days (mean = 8). Data were available for analysis from 68 of them, aged 18 to 84 years (mean = 44). The "taking compliance" for the whole story group, which corresponded to the ratio of the number of times the bottle was opened and the total number of doses prescribed during the monitoring period, was nearly perfect at 99.6%. However, only 32.6% of the medications was taken within 1 hour before or after the 12-hour interval expected to be optimal for a twice-daily regimen. It therefore seems highly desirable that physicians give more detailed recommendations to their patients regarding the drug regimens they prescribe.