Fasting serum cholesterol, triglycerides, thyroxine, triiodothyronine, uric acid, gastrin, and insulin were measured in a group of 24 Muslims at the beginning and end of Ramadan--the Muslim month of fasting. There was a significant increase in the levels of total serum cholesterol, thyroxine, and uric acid and a significant fall in body weight. There was no significant change in the levels of total serum triglycerides, triiodothyronine, gastrin, insulin (fasting), or in the rise in gastrin or insulin 1/2 h after food. These changes, although unlikely to affect normal people, may be significant in patients. Physicians working in Muslim countries should be aware that Ramadan may affect some laboratory findings.