To evaluate the degree of compliance with pharmacological therapy, and to identify predictors of non-compliance in outpatients from a cardiology referral center in São Paulo, Brazil, we studied 485 outpatients 230 (47.4 percent) males and 255 (52.6 percent) females, through an interview guided by a questionnaire during medical consultation. The ages ranged between 17 and 86 (mean 54, standard deviation 15) years. Heart disease and socioeconomic factors (residence, means of transport, educational level and professional status) were studied. In addition, we examined the drugs prescribed including: difficulties in taking them; the source of supply, and the patient's knowledge of the drugs. Assessment of compliance was based on the patients' response. The patients' answers were compared with the prescription and progress notes. Errors were recorded if the patient reported using one or more nonprescribed medicines. Compliance with therapy was recorded if the patient said the prescription was taken correctly without interruption and without error. The variables with significant differences in univariate analysis were further analyzed by multivariate log-linear regression analysis. Noncompliance occurred in 286 (59 percent) of the patients, and was predicted by the reported difficulty in taking medication (P < 0.001), and by the lack of knowledge of medication names (P < 0.001). Thus, noncompliance with medical therapy was common. The main predictors of non-compliance were the reported difficulty in taking medication and inability to identify medicines' names.