Growth and weight gain were studied longitudinally over a period of four years in thirty-nine children treated for acute lymphoblastic leukemia. The children were divided into two groups according to treatment. Twenty-eight children were given prophylactic cranial irradiation and eleven children were treated without such irradiation. The duration of cytostatic treatment was three years in all cases. Average growth during the first two years was similar in the two groups, and the standard deviation scores (SDS) were below average. The rate of growth (in height) during the fourth year was significantly higher among those children who had not received cranial irradiation (p less than 0.01). After four years the average attained height had declined 0.5 SD for children treated with cranial irradiation and 0.2 SD for children without such treatment. Weight velocity was significantly greater than the expected mean in the non-irradiated group during the first year and in the irradiated group during the fourth year of the study. Attained weight after four years had increased 0.4 SD more among those children who had not received irradiation. The results suggest that prophylactic cranial irradiation is responsible for the greater part of the prepubertal growth inhibition in these children.