Delirium syndrome is common in the hospitalized elderly population. However, data on its numerous etiological factors are scarce. Clinical observations suggest that delirium incidence could be related to seasonality. In order to evaluate the seasonal influence on the incidence of delirium syndrome among the patients of a geriatric hospital, we performed a retrospective study reviewing the medical records of 5218 patients aged 65 years and older, who were hospitalized in three medical wards between the years 1991 and 1997. The diagnoses of delirium were made according to ICD-9-CM. Of the 4929 subjects enrolled, 3548 (72%) were referred for assessment to the psychiatric or neurologic specialist, because of acute changes in their mental or behavioral status. Of those patients, 312 (6.3%) were diagnosed with delirium at admission, and 234 (4.7%) developed delirium during their hospitalization. Statistical analysis of incidence revealed a seasonal variation, with higher rates in the winter than in the summer months (P<0.001). This study suggests a seasonal influence on delirium syndrome incidence and a possible etiological relation with seasonal factors like the light -dark cycle.