BACKGROUND: Previous studies suggested a higher incidence of acute heart failure (AHF) during cold months in regions with cold climate. We examined the daily incidence of AHF by same-day trough temperature and humidity in a warmer region. METHODS AND RESULTS: All admissions for AHF (340 patients) to a city hospital, providing the sole emergency medical care to a geographical region of approximately 500,000 people were recorded. Patients were followed through admission and for 6 months after discharge. Low minimal trough temperature was associated with an increase in the same-day AHF incidence. Lowest tercile trough night temperatures were associated with higher AHF incidence (3.5 +/- 2.1 versus 2.4 +/- 1.6, events/24 hours, P = .012). This association was mainly from increased AHF events in nights with the predetermined trough temperature of