The effect of high estrogen doses on weight was studied in 36 adolescent girls with familial tall stature treated to reduce adult height. Mean weight gain during the first year was 9.03.6 kg. Thereafter, there was no or minimal gain on continued treatment. The largest weight velocity occurred during the first 6 months. Within that period, it was most marked during the first 5 weeks, probably due to early water retention. The gain (total, in different groups of patients and in treatment periods of different duration) did not correlate with height and weight before treatment expressed in absolute values or standard deviation scores. It is concluded that the weight gain induced by long-term estrogen treatment cannot be predicted quantitatively before treatment in individuals, and that heavy or fat girls do not necessarily gain more weight than light and lean girls.