Body weight development during pregnancy was monitored for 2295 women, and up to 1 year post-partum for 1423 of them, at 14 maternity clinics throughout Stockholm. The objective was to find predictors for post-partum weight retention. The mean weight gain after 1 year post-partum compared with the pre-pregnancy body weight (delta-weight) was 1.5 +/- 3.6 kg (P less than 0.001). Of the group 30 per cent lost weight, 56 per cent gained 0 to less than 5 kg and 14 per cent gained greater than or equal to 5 kg. When this result was corrected for possible average underestimation of the self-reported pre-pregnancy weight and weight gain with age, the mean delta-weight was 0.5 kg. The factor with the highest correlation with delta-weight was pregnancy weight gain (r = 0.36, P less than 0.001). Very low r-values, although statistically significant, were obtained for the correlation between the delta-weight and lactation (r = -0.09, P less than 0.01) and age (r = 0.06, P less than 0.05). The delta-weight was not correlated with pre-pregnancy body weight or parity. Women with a delta-weight of greater than or equal to 5 kg had, on average, a higher pre-pregnancy body weight, but initially overweight women had a more variable weight development than lighter women. One in every four women with a weight retention of greater than or equal to 6 kg after a previous pregnancy experienced a high weight gain even after the present pregnancy. Women who stopped smoking had a significantly higher delta-weight than either smokers or non-smokers. Women in the age group greater than or equal to 36 years had a higher mean pre-pregnancy body weight than younger women, and in the age group 26-35 years the pre-pregnancy body weight was increased with increased parity. Thus, post-partum weight development is individual and of the factors studied here only high weight gain during pregnancy and smoking cessation can be considered as predictors for persistent weight gain after 1 year post-partum.