A study of the seasonal variation of migraine. Available evidence supports the contention that migraine involves a disturbance in serotonin function. Several parameters of serotonin function in humans have been found to vary seasonally and may underlie the seasonal fluctuations observed in many clinical neuropsychiatric phenomena that are thought to involve serotonin dysfunction. We therefore postulated that migraine headaches might also vary seasonally and examined the admissions to our hospital over a 20-year period with a primary diagnosis of migraine. Peak admissions were found to occur most frequently in the spring for females in comparison to males (p less than or equal to 0.04, chi-square). The implications of these findings are discussed. 