Our objective was to determine if Chinook weather conditions in the Calgary area increase the probability of headache attacks in migraine sufferers. Environment Canada meteorologic summaries for January through June 1992 were analyzed and times of Chinook wind onset identified. Chinook weather conditions were defined as calendar days when Chinook winds were present and the calendar day immediately preceding Chinook wind onset. The diaries of 13 migraine patients were analyzed, and times of headache onset classified according to the existing weather conditions. The probability of migraine headache onset was greater on days with Chinook weather (17.26%) than on non-Chinook days (14.65%) (P = 0.042). Older patients appeared more weather sensitive than younger patients. For patients over age 50, the probability of migraine occurrence on Chinook weather days was much greater than on non-Chinook days (P = 0.007). Chinook weather conditions increase the probability of migraine headache occurrence. Older migraine sufferers appear particularly vulnerable to this effect.