Aims Patients with primary Raynaud's phenomenon (PRP) have more severe symptoms in the winter. The aetiology of this is more complex than simply increased vasoconstriction in response to the immediate ambient temperature. The aim of this study was to investigate differences in skin temperature (Tsk), microvascular blood ow and responses to endothelium-dependent and independent vasodilators in healthy controls, and women with PRP under identical environmental temperatures but in different seasons. Methods Ten women with PRP were compared with age matched women (10) and men (10). Finger skin responses were recorded immediately on arrival, after stabilizing in a temperature regulated laboratory at 22±24u C, and at matched warm (35u C) and cold (15u C) Tsk in the winter and summer. Baseline red blood cell¯ux (r.b.c.¯ux), and the change in¯ux in response to iontophoresis of acetylcholine (ACh) and sodium nitroprusside (SNP) were recorded by laser Doppler¯uxmetry at the warm and cold Tsk. Results Arrival Tsk were signi®cantly cooler for all subjects during the winter (mean seasonal difference x2.6u C, P<0.0001), and markedly colder in subjects with PRP (mean seasonal difference x3.5u C, P<0.0005). Statistically signi®cant seasonal differences persisted in all subjects at stable Tsk despite an identical laboratory temperature (mean difference 1.3u C, P<0.0001). To achieve comparable controlled ®nger Tsk a signi®cantly colder local environment was required for male controls (mean of x2.1u C, P<0.0001), and a signi®cantly warmer environment for subjects with PRP (mean of + 2.4u C, P<0.0001) compared with female controls. This needed to be warmer in the winter, by a mean of 2.4u C, than the summer for all subjects. Vasodilatation in response to ACh, but not SNP, was signi®cantly smaller (P<0.0001) in the PRP group compared with the female controls for all visits, with most of this difference arising in the winter visits (P<0.01). Conclusions There is a seasonal and persistent in¯uence on ®nger Tsk, and microvascular blood¯ow in healthy men and women, which modi®es the observed responses to immediate changes in ®nger Tsk. The seasonal differences are greater in women than men, and are further exaggerated in women with PRP, in whom this is associated with reduced endothelium-dependent vasodilatation.