To describe the relationship between mood and body weight, we analyzed 122 consecutive days of data from a 52-year-old woman with rapid cycling bipolar disorder, type II. The patient completed a daily log of self-reported mood, and weighed and recorded her weight daily. Mood and body weight were highly correlated (r = 0.60), and a decrease in mood preceded a decline in body weight by 3 days. Reports of loss of appetite on the Hamilton Depression Rating Scale also corresponded to periods of weight loss. There was a negligible difference in the patient's resting metabolic rate between a period of depression (1628 kcal/day) and of euthymia (1724 kcal/day). The association between depression and weight loss was primarily mediated by decreased food intake. Such patients should provide an excellent opportunity to study the regulation of body weight with affective illness.