We studied the effect of phototherapy on body weight in 4 overweight women. Melatonin was measured in the serum and urine before and after 1 hr of bright light (350 cd/m2). Psychiatric self-ratings with the Comprehensive Psychopathological Rating Scale (CPRS) and Visual Analog Scale (VAS) were performed. Phototherapy (1,500 lux) was given daily at 7-9 a.m. for 10 days and thereafter twice weekly for another 4 1/2 weeks. Three of the 4 women reduced their net weight (1.5-2.4 kg) and improved in mood. All were sensitive to light. The findings indicate that phototherapy affects the melatonin-serotonin system and carbohydrate regulation and may reduce body weight.