The influence of weather on the onset of paroxysmal atrial fibrillation (PAF) was studied in 1099 patients who were admitted to the Outpatient Clinic of the Department of Internal Medicine Split during the period 1981-1987. The cumulative frequencies of PAF during bora, sirocco and calm weather were determined. There were 870 bora or sirocco days when the wind lasted at least 8 hours at the velocity exceeding 20 km/h. Thirty two windy days that were neither bora nor sirocco were not studied. The rest of 1654 days were considered as calm weather. The incidence of PAF during 1654 calm days (0.41/day) was significantly different from PAF incidence during 440 days with sirocco (0.50/day, p < .05), but not from the incidence of PAF during 430 days with bora (0.46/day, p < .05). Logistic regression analysis, and control of the patients ages, revealed a significant correlation between type of the wind of various intensities and onset of PAF (chi 2 = 12.73, d.f. = 6, p < .05). The greatest contribution to this correlation was exerted by sirocco in patients over 50 years of age.