A randomized, double-blind, placebo controlled, cross-over study was conducted to determine the clinical efficacy of and patient tolerance to sustained-release glyceryl trinitrate (GTN) patches in the treatment of Raynaud's phenomenon. The patients studied were those with primary Raynaud's disease (n = 21) and patients with Raynaud's phenomenon secondary to systemic sclerosis (n = 21). GTN patches (0.2 mg/h) were found to be effective in reducing the number (P < 0.05) and severity of Raynaud's attacks (P < 0.05) in both groups of patients. Objective assessments by infrared thermography did not show any significant improvement when the patients were on GTN patches (P > 0.05). Headaches, refractory to treatment, led to the withdrawal of eight patients from the study and occurred in approximately 80% of the remaining patients. Thus, in spite of the subjective efficacy, the frequent headaches will limit the use of GTN patches in Raynaud's phenomenon. Elucidating the mechanism of action of the beneficial and adverse effects of GTN at the molecular level may shed light on the pathogenesis of Raynaud's phenomenon and its treatment.