Several cardiovascular diseases (CVDs) exhibit seasonal variations in their incidence. Although seasonal variation in CVD is largely driven by predictable changes in weather conditions, a complex interaction between ambient environmental conditions and the individual is evident [1]. Seasonality in the incidence of CVDs could reflect variation in meteorological characteristics, environmental factors, background rates of respiratory and other infectious illnesses, and lifestyles factors. The winter and summer seasons are associated with higher and lower incidence for acute CVDs such as acute heart failure (AHF), acute myocardial infarction (AMI), and acute aortic dissection (AAD), respectively [2], [3], [4], [5], [6], [7]. However, there is lack of evidence of how seasonal variation translates to in-hospital outcomes among acute CVDs in a contemporary nationwide assessment [8]. In addition, the impact of potential cofounders such as age, sex differences, and clinical backgrounds on inpatient outcomes in this seasonal variation is not fully evaluated enough. Appreciating the seasonal variability in hospitalization and inpatient outcomes has the potential to improve quality of care and patients’ outcomes. Therefore, we investigated whether the acute cardiovascular diseases such as AHF, AMI, and AAD have distinct seasonal variations in the number of hospitalizations and in-hospital mortality using a contemporary nationwide database in Japan. We hypothesized that the seasonal effects on the in-hospital mortality may be different between at early phase and late phase of hospitalization in the acute CVDs within a month and analyzed in-hospital mortality by dividing to early and late mortality. We also clarified that clinical confounders such as age, sex, and clinical background may have significant impact on the seasonal variations.