Pregnancy constitutes a biological cause of weight cycling. Since repeated weight variation has been associated with sustained weight increase and subsequent metabolic complications, the role of pregnancy in affecting body weight was analysed retrospectively in a group of 128 severely obese patients at the obesity unit. Their present mean age was 47.8 +/- 10.7 years, the mean age at birth of their first child 24.1 +/- 5.1 years and their mean present BMI was 37.8 +/- 5.5 kg/m2 (s.d.). Seventy-three percent of the women reported a weight retention of more than 10 kg one year after delivery. The median number of other weight cycling events was seven. Such cycling did not correlate to present BMI or weight increase during any pregnancy. Weight increases during subsequent pregnancies were not correlated to each other. The results support the hypothesis that for women who develop severe obesity, their pregnancy-related weight increases contribute substantially to their body weight development.