In a randomized cross-over double-blind study of 30 out-patients with true angina the efficacy of fendiline and isosorbide dinitrate (ISDN) was compared. In randomized order the patients daily took either 3 X 50 mg fendiline or 2 X 20 mg ISDN (plus 1 placebo tablet). Each treatment period lasted for six weeks. Ergometric tests were performed at the onset of the study and at intervals of three weeks. The main criterion for the efficacy of the treatment was a change in S-T segment (at 0.06 s after the QRS). With either drug there occurred a statistically significant reduction in S-T depression and in the frequency of anginal attacks during the first treatment period. But there was a difference between the two drugs: Whereas with ISDN a marked reduction in S-T segment depression was observed after three weeks, this effect slightly receded in the second part of the treatment period, while with fendiline the reduction in S-T depression continued over the entire treatment period. There was no significant difference between the two drugs as to side-effects or tolerance.