Recent hypotheses suggest that for women who develop morbid obesity, increases in weight associated with pregnancy may represent a significant contribution to their obesity status. The effects of multiple pregnancies on weight gain were studied in 96 morbidly obese women (< 13.6 kg over ideal weight at ages 20-24 or before an earlier first pregnancy and currently > 44.5 kg over ideal weight) and 115 random control women from the Utah population. Self-reported weights for each pregnancy included: prepregnancy, greatest during pregnancy, and 6 weeks following delivery, which were validated against available hospital records. Mean number of pregnancies in each group were similar (4.2 and 4.3), ranging from 1 to 9. Mean current age was 46 and mean weight gain since ages 20-24 was 46.0 kg in the morbidly obese and 14.1 kg in controls. Regression of current weight on total number of pregnancies, adjusting for weight at ages 20-24, showed a 1.3 kg/pregnancy increase in current weight (p = 0.03) with no difference between groups (p = 0.6). Weight gain subsequent to the last pregnancy was not related to the number of pregnancies (p = 0.2). Morbidly obese women gained more weight during pregnancy than controls only for the first pregnancy. Gains were similar for all other pregnancies. Morbidly obese women had smaller weight losses after delivery than the controls, but these differences were not significant. For the first pregnancy, morbidly obese women had a net weight retention that was 4.0 kg greater than the controls at 6 weeks post-partum and an average of 1.6 kg/pregnancy greater retention for the remaining pregnancies. Pregnancy weight gains for each pregnancy subsequent to the first pregnancy were constant. These findings suggest: 1) women who develop morbid obesity have slightly less weight loss after delivery and greater between-pregnancy weight gains than controls; 2) the number of pregnancies does not affect the amount of weight gained after the last pregnancy; and 3) while multiparity may augment weight gain in morbidly obese women, it is probably not a primary factor in the later development of morbid obesity.