Introduction:
Acute coronary syndromes (ACS) are the leading cause of death all over the world. In the last years, the chronobiology of their occurrence has been changing.

Material and methods:
Medical records of 10,529 patients hospitalized for ACS in the Medical University of Bialystok, in 2008–2017, were examined. Weather conditions data for Bialystok County were obtained from the Institute of Meteorology.

Results:
: The highest seasonal mean for ACS was recorded in spring (OR = 1.08, 95% CI: 1.00–1.18, p = 0.049) and it was the season with the largest temperature changes from day to day (∆ temp. = 11.01). On the other hand, every 10ºC change in temperature was associated with increased admission due to ACS by 13% (RR = 1.13, 95% CI: 1.040–1.300, p = 0.008) and 12% in patients over 70 (RR = 1.118, 95% CI: 1.001–1.249, p = 0.048, lag 1). Analysis of weekly changes showed that the highest frequency of ACS occurred on Thursday (OR = 1.16, 95% CI: 1.05–1.28, p = 0.003), while in the STEMI subgroup it was Monday (n = 592, mean = 0.94, SD = 1.04, OR = 1.20, 95% CI: 1.07–1.36, p = 0.003). Sunday was associated with decreased admissions due to all types of ACS (OR = 0.70, 95% CI: 0.63–0.77, p < 0.001).

Conclusions:
Weather conditions have an impact on ACS frequency and the elderly are more susceptible. We observed a shift in the seasonal peak of ACS occurrence from winter to spring which may be related to temperature fluctuations associated with climate change in this season. The lowest frequency of ACS took place on weekends.