BACKGROUND: Understanding disease seasonality is important for improving clinical practice, hospital resource utilization and community-based preventive care. However, no studies have investigated the seasonality of acute kidney injury (AKI). METHODS: In the Tokushukai Medical Database, which includes 38 Japanese community hospitals, we identified hospitalized patients with AKI based on the Kidney Disease: Improving Global Outcomes serum creatinine criteria from January 2012 to December 2014. We plotted the number and proportion of patients with AKI among hospitalized patients by month of hospital admission. Subgroup analyses were conducted by the admission diagnosis category, timing of AKI diagnosis and age. We also examined the association between month of hospital admission and AKI, adjusting for patient characteristics and AKI risk factors. Finally, we assessed seasonal variations in disease severity and 30-day mortality of patients with AKI. RESULTS: We identified 81 279 (14.6%) patients with AKI among 555 940 hospitalized patients. The proportion of patients with AKI was highest in January (16.7%) and lowest in June (13.4%). Subgroup analyses suggested that the seasonality of AKI incidence was driven by community-acquired AKI associated with the admission diagnosis of cardiovascular and pulmonary diseases among older patients. The adjusted odds ratio for AKI (January versus June) was 1.24 (95% confidence interval, 1.17-1.31). Patients with AKI showed a larger number of failing organs in winter, and their 30-day mortality was 16.4% in spring, 14.5% in summer, 15.6% in autumn and 18.4% in winter. CONCLUSION: AKI is more common among hospitalized patients and patients with AKI are more severely ill in winter.