OBJECTIVES: We hypothesized that people in Morocco are more irritable during the month of Ramadan than during the rest of the year. Our objectives were to measure irritability in fasting Muslims during the month of Ramadan, to describe its various modes of expression, and to examine risk factors for this irritability. METHODS AND SUBJECTS: We studied 100 healthy volunteers during the month of Ramadan for two successive years (1994 and 1995). All subjects were male (mean age, 32+/-5.8 years), and 51% of them were smokers. Irritability was assessed over a 6-week period (before, four times during, and after the end of Ramadan). We assessed both subjective (visual analog scale) and objective irritability. We also recorded the consumption of psychostimulants, duration of sleep, and anxiety level as measured by the Hamilton Anxiety Scale. RESULTS: Irritability was significantly higher in smokers than in nonsmokers before the beginning of Ramadan. It was higher in both groups during the Ramadan month. Irritability increased continuously during Ramadan and reached its peak at the end of the month. Consumption of psychostimulants (coffee and tea) and anxiety level followed the same pattern. Smokers and nonsmokers had a similar pattern of irritability over time, but irritability increased more in smokers than in nonsmokers.