The prostaglandin thromboxane A2 causes platelet aggregation and vasoconstriction and may be important in the pathogenesis of Raynaud's phenomenon. Therefore, a randomized, double-blind, placebo-controlled trial was conducted to assess the effectiveness of dazoxiben, a selective thromboxane synthetase inhibitor, in the treatment of Raynaud's phenomenon and to compare it with nifedipine, a calcium channel blocker. Twenty-two subjects who had at least one episode of Raynaud's phenomenon per day entered the study. Three patients withdrew from the study because of side effects while taking nifedipine. There was no difference among the subjects' subjective evaluation of the three treatments. Seven of 19 (44 percent) reported a moderate to marked improvement while taking placebo compared with 12 of 19 (63 percent) taking nifedipine and five of 19 (26 percent) taking dazoxiben (p = NS). Similarly, there was no difference in the mean two-week episode rate among the three treatments: placebo 30.4 +/- 4.5, nifedipine 24.7 +/- 5.6, dazoxiben 32.0 +/- 4.9 (p = NS). Twelve of 22 subjects experienced side effects while taking nifedipine as compared with two of 21 taking placebo and eight of 21 taking dazoxiben (p less than 0.005). These data show that dazoxiben is not effective in the treatment of Raynaud's phenomenon and suggest that thromboxane does not cause the vasoconstriction that characterizes this disorder.