The efficacy of prazosin (Minipress) for the treatment of Raynaud's phenomenon (RP) was assessed by patient diary data and by objective measurements of digital vessel patency before and after cold challenge. Nine of the 14 patients with complete data reported fewer RP episodes during the 2 weeks in which they received prazosin than with placebo. Eight of 9 patients in Group 1 (systemic lupus erythematosus, mixed connective tissue disease or idiopathic RP) reported improvement during prazosin therapy (p less than 0.05) while only one of 5 patients with progressive systemic sclerosis (PSS, Group 2) reported benefit. Objective measurements indicated a significant decrease (p less than 0.02) in digital artery resistance with prazosin only in Group 1. These data predict that prazosin may be beneficial in the treatment of RP for patients who do not also have PSS.