A total of 126 patients from general practice with chronic stable angina pectoris entered the treatment phase of this open, randomized, crossover comparison of 20 mg isosorbide-5-mononitrate, and 20 mg nifedipine. Both treatments were given orally, three times daily, for 4 weeks and sublingual administration of glyceryl trinitrate was allowed throughout. Over the whole treatment period, there was no statistically significant difference between treatments for anginal attacks. However, significantly fewer glyceryl trinitrate tablets were required by patients receiving prophylaxis with nifedipine, although this difference was too small to be of clinical significance. No statistical difference existed between treatments in respect of scores for 'overall intensity of pain', 'physical exercise ability' and 'general well-being'. Of those patients who expressed a preference, the majority preferred the second treatment with no statistically significant difference between isosorbide-5-mononitrate and nifedipine. Both treatments showed similar levels of adverse events, the major difference (not significant) being for flushing of the skin which occurred in five patients given nifedipine compared with one patient given isosorbide-5-mononitrate. It is concluded that, in clinical terms, the two treatments were similar. Headache and dizziness/giddiness were the most frequently recorded adverse events.