OBJECTIVES: To identify enhancing factors of the medication adherence using the data from a randomized trial of patients with chronic heart diseases. METHODS: We used the data from a randomized trial involving a total of 6967 patients with chronic heart diseases. This trial followed up them with an average of 5 years. Adherence rate was obtained by the division of a number of days to take a targeted medicine by 365 days each year for each patient. The targeted medicine was the one assigned by randomization. An overall adherence rate for each patient was defined by an average of annual adherence rates during the follow-up period. Good medication adherence was defined by the overall adherence rate of 80% or more. An adjusted odds ratio (OR) was used to show the degree of influence to a good adherence. RESULTS: Very high rate (94%) of good adherence was observed since the medication was an assigned drug in a randomized trial. Enhancing the odds of good adherence was women (OR=1.13, P=0.007), age (OR per 5-year increase =2.33, P=0.17), patients taking 5 or more medicines (OR=1.45, P<0.001), and good status on diet therapy (OR=1.95, P<0.001). Conversely, smokers decreased the adherence by 12% (P=0.008) and patients with previous stroke decreased the adherence by 29% (P<0.001). CONCLUSIONS: Patients taking many drugs or receiving diet therapy and women showed a statistically significant increase in the medication adherence.