BACKGROUND: Blood pressure (casual and daytime) is higher in winter than it is in summer, and this factor might be partly responsible for the higher cardiovascular mortality in winter. OBJECTIVE: To determine in a prospective study whether there is also a seasonal variation in exercise blood pressure. METHODS: We evaluated the pretest, exercise (five-step treadmill test) and recovery values of systolic blood pressure (SBP), diastolic blood pressure (DBP), and heart rate of 94 healthy men, aged 20-65 years, during the summer and winter. RESULTS: SBP and DBP values were higher in winter than they were in summer during the resting period immediately before exercise (P = 0.003 and P = 0.07, respectively), and during recovery (both P < 0.0001). Exercise SBP and heart rate values were similar in the two seasons, although exercise DBP was higher in winter than it was in summer (P < 0.0001). The increase in DBP from pretest to stage 3 was 6.5 mmHg in summer and 10.4 mmHg in winter (P = 0.002). After we had controlled for possible confounders, the increase in DBP during exercise was found to be independently associated with season of the year and resting DBP. CONCLUSIONS: Exercise DBP varies according to season whereas exercise SBP does not. Our results suggest that, since measures of the response of blood pressure to exercise testing are frequently used in blood pressure and hypertension research as well as in clinical practice, the seasonal influences should be taken into account.