This study was designed to estimate the frequency and characteristics of headaches occurring on the first day of Ramadan (Moslems' fasting month) and to determine possible causes. One hundred fifty copies of a specially designed questionnaire were distributed on the second day of fasting to a random sample of hospital staff. Completed questionnaires were obtained from 116 subjects (77%). Headaches were reported by 37 (41%) of the 91 persons who had fasted as compared to 2 (8%) of those 25 who did not fast (P=.002). The headache was of tension type in 78% of the cases. Headache frequency increased with the duration of fasting and affected mainly those prone to have headaches, more particularly of the tension type and the most important exogenous-associated factor was caffeine withdrawal. Other factors such as lack of sleep, hypoglycemia, and dehydration may have been contributory in a small number of cases. A progressive reduction of caffeine consumption in the weeks preceding the month of Ramadan and a cup of strong coffee just before the start of the fast may prevent the occurrence of first-of-Ramadan headache.