BACKGROUND: The aim of this study was to clarify the relationship between onset of acute myocardial infarction (AMI) and weather conditions, to determine whether days in which AMI onset is likely can be predicted. METHODS and RESULTS: Of the 929 patients admitted to our hospitals in Kagoshima prefecture with AMI, subjects comprised 611 patients. Days of frequent onset (F-days) were defined as days with > or Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 3 patients/day admitted for AMI, with days of non-frequent onset (N-days) defined as days with < 3 patients/day. Meteorological factors were measured, and daily differences in all parameters and intraday temperature differences on the onset day, and 1 and 2 days before onset were calculated. F-days were significantly associated with intraday temperature differences on the onset day (10.3 degrees C vs. 7.9 degrees C, pEuro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin)0.005), 1 day before onset (10.7 degrees C vs. 7.9 degrees C, pEuro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin)0.002), and 2 days before onset (11.3 degrees C vs. 7.9 degrees C, pEuro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin)0.0001). A cutoff intraday temperature difference of > or Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 9.4 degrees C on 1 and 2 days before onset was predictive of F-days with 89% sensitivity and 87% specificity. CONCLUSIONS: Intraday temperature differences offer a powerful predictor of F-days. Onset of AMI can be predicted based on weather conditions over the preceding 1-2 days.