OBJECTIVES: This study was done to determine whether seasonal variation exists in hospitalizations and deaths due to heart failure (HF) and to examine possible contributors to such variability. BACKGROUND: Although seasonal variation in the incidence of acute myocardial infarction and sudden death is well recognized, it is less well documented in HF. METHODS: We used the linked Scottish Morbidity Record scheme, which provides individualized morbidity and mortality data for the entire Scottish population. RESULTS: Between 1990 and 1996, there were a total of 75,452 male and 81,269 female hospitalizations related to HF in Scotland, with an average rate of admissions per 100,000 population of 8.4 and 8.5 per day, respectively. Significantly more admissions occurred in winter compared to summer (p < 0.0001). In women, the peak rate of admission occurred in December (12% more than average) and the lowest rate in July (7% less than average) (odds ratio [OR] 1.14, p < 0.001). The respective figures for men were 6% more, 8% less (OR 1.16, p < 0.001). In both genders, the greatest variation occurred in those aged >75 years - peak winter rates being 15% to 18% higher than average. There was also a winter peak in concomitantly coded respiratory disease; this seasonal excess accounted for approximately one-fifth of the winter increment in HF hospitalizations. Seasonal variation in mortality was also seen in these patients. The number of male deaths in December was 16% higher, and in July 7% lower, than average (OR 1.25, p < 0.001). In women, the equivalent figures were 21% higher (January) and 14% lower (July) (OR 1.21, p < 0.001). Again, the greatest variation occurred in those aged >75 years - peak rates being 23% to 35% higher than average. CONCLUSIONS: There is substantial seasonal variation in HF hospitalizations and deaths, particularly in the elderly. Approximately one-fifth of the winter excess in admissions is attributable to respiratory disease. Extra vigilance in patients with HF is advisable in winter, as is immunization against pneumococcus and influenza.