Thirteen sedentary adult females successfully quit smoking cigarettes for 48 days. Mean daily caloric consumption increased 227 kcal and mean weight gain was 2.2 kg. There were no measurable acute effects of smoke inhalation and no chronic net effects of smoking cessation on resting metabolic rate, as determined by oxygen consumption and respiratory exchange ratio. After 1 yr, subjects who continued to abstain gained an average of 8.2 kg. HDL-cholesterol increased 7 mg/dl in 48 days; however, this effect was lost in those who returned to smoking. Increased caloric consumption accounted for 69% of weight gained immediately following smoking cessation. Factors other than changes in caloric consumption and metabolic rate may be responsible for a significant proportion (31%) of the weight gained in individuals who quit smoking.