The records of 377 bipolar disorder patients who were consecutively admitted to a general inpatient psychiatric unit in mid-Michigan over a 6-year period were examined. The seasonal variation of hospitalization, total sleep time, thyroid stimulating hormone, creatinine levels, lithium dosage and serum levels, aggressive behavior, and treatment outcome were analyzed. Among men, the admission rate peaked in the springtime. Women demonstrated a bimodal season distribution, with peak admission rates in spring and fall. Aggressive behavior in both men and women peaked in the spring (z = 2.50, p < 0.05). Men maintained on lithium achieved higher serum lithium levels during the summer months. These findings parallel previous reports regarding the influence of seasons upon bipolar disorder. The therapeutic implications related to seasonality and mania are discussed.