Spontaneous growth was analysed in a group of 55 girls with Turner's syndrome and various karyotypes. Their variation in height and its dependence on parental height were similar to that of normal girls. At all ages, the 45,X karyotype was associated with slightly greater mean stature than the other karyotypes together. The bone ages lagged progressively behind from 10 years onwards. Twenty-five patients aged between 9.1 and 17.2 years were given fluoxymesterone, 0.06-0.17 mg/kg daily, for at least 1 year. Their height velocities increased significantly. This brought about a clear psychological benefit. Their final heights were predicted before and after therapy, with a new method based on the spontaneous growth and bone maturation of our patients. The response was individually variable but, on average, the patients gained in predicted height from the therapy. This effect was not lost during a posttreatment year. Abnormal lowering of the voice occurred in patients receiving greater than or equal to 0.15 mg/kg fluoxymesterone daily, but never with less than 0.13 mg/kg. No other adverse effects appeared. Thus, fluoxymesterone is useful for promoting growth in girls with Turner's syndrome.