BACKGROUND Ramadan is the holiest month in the Islamic calendar and Muslims fast during this month. We designed this study to evaluate the effect of Ramadan fasting on plasma lipids and lipoproteins. METHODS This cohort study was performed during Ramadan in December 2002 (Islamic year 1423). The subjects were 81 students of Tehran University of Medical Sciences. We evaluated weight, body mass index (BMI), glucose, triglyceride (TG), cholesterol, low density lipoprotein (LDL), high density lipoprotein (HDL), and very low density lipoprotein (VLDL), before and after Ramadan. RESULTS Body weight and BMI both decreased during Ramadan fasting in both genders. Glucose and HDL decreased and LDL increased significantly during fasting in Ramadan, but there was no significant change in total cholesterol, TG and VLDL. We did not find any association between TG, cholesterol, LDL, VLDL, HDL and the following variables: sex, body weight changes, and two or three instances of meals before Ramadan. Triglyceride level also increased in students with normal BMI while it decreased in overweight subjects. CONCLUSIONS This study indicated that Ramadan fasting led to a decrease in glucose and weight. Although there was a significant reduction in meal frequency, a significant increase in LDL and decrease in HDL was noted during Ramadan. It seems that the effect of Ramadan fasting on serum lipid levels may be closely related to the nutritional diet or biochemical response to starvation.