This study aimed to clarify the association between the onset of acute coronary syndrome (ACS) and weather conditions in summer and winter in the same region. At a general hospital inJapan, weather conditions during the onset of 2,381 cases diagnosed with ACS over 25 years were analyzed using a generalized additive model adopting log-quasi-Poisson distribution as the link function, with the occurrence of ACS as the dependent variable and weather data as the independent variable. In conclusion, we found that ACS occurred at about the same frequency in winter and summer, and the season did not affect the onset. The onset rate decreased 0.960-fold per 1 °C increase in the minimum temperature one day before the day of onset and decreased 0.987-fold per 1 hPa increase in the mean station pressure of the previous day.