Our previous studies have suggested that digital blood pressure response to cooling could provide a measure of the efficacy of treatments that are administered to patients with Raynaud phenomenon (RP). This method was used on 158 primary RP patients participating in a multicenter, randomized clinical trial that compared the efficacy of sustained-release nifedipine with temperature biofeedback in the treatment of RP. A pill placebo and electromyography served as controls. The response to local finger cooling was measured at 30 degrees, 20 degrees, 15 degrees and 10 degrees C in a temperature-controlled room under standardized conditions. The results showed that, at the 15 degrees C and 10 degrees C local cooling temperatures, the patients in the nifedipine group had a higher mean digital systolic blood pressure, a higher relative digital systolic blood pressure (RDSP), a smaller proportion of subjects with RDSP < 70% and a smaller proportion of subjects with a zero reopening pressure than the patients in the three other treatment groups. These results were statistically significant at 10 degrees C, the nifedipine group being significantly different from all others (p < 0.05); no significant difference was found between the three other treatment groups.