Eighteen patients, 13 male and five female, with unstable angina pectoris and who had greater than or equal to 3 anginal attacks/day despite treatment with beta- and/or Ca-blockers were treated for three days with transdermal nitroglycerin (Transiderm-Nitro 25 mg/24 hours) in a randomised, placebo-controlled trial. A significant reduction of anginal attacks (p = 0.03) was demonstrated only during the first day of treatment. The lack of significant effects during the following two days was ascribed partly to the possible development of nitrate-tolerance, partly to a declining number of anginal attacks in the placebo-group during the trial period, for which reason the difference between the actively treated and the placebo group became smaller and finally the consumption of nicomorphine was considerably greater in the placebo group.