The literature was reviewed in an effort to relate frequency of dosing and other influences with patient compliance in medication taking. Once-a-day and twice-a-day regimens were associated with significantly better compliance (73% and 70%, respectively) than were three-times-daily (52%) and four-times-daily (42%) regimens. Compliance is not related to income, social class, occupation, or educational background, and it cannot be accurately predicted by physicians. Unintentional errors in taking medication are made by 50% to 90% of patients.