A previous study showed that when GH-deficient children below the third percentile in height are treated with 0.168 U human GH (hGH)/kg BW3/4 for 10 days, their height increases by 0.3--1.9 cm during the next 8 weeks. The present study determined whether this acute response would predict the child's long term response to 1 yr of treatment with the same dose of hGH given three times a week. Eighteen GH-deficient children and adolescents, aged 8--16 yr, were measured every 2 weeks over 108 weeks. After a control period of 12 weeks (period 1), the patient received hGH for 10 days. During the remainder of the 12 weeks of period 2 and during the next 12 weeks (period 3), hGH was not given. Patients recieved hGH three times a week during periods 4 and 5 (24 weeks each). Periods 6 and 7 (12 weeks each) were posttreatment control periods. During periods 1, 3, 6, and 7, rate of growth was less than 0.2 cm/month. During period 2, the rate ranged between 0.1--0.8 cm/month. During periods 4 and 5, the growth rate ranged from 0.2--1.0 cm/month. Rate of growth during periods 4 and 5 (y) was related to rate during period 2 (x) by the equation y = 0.027 + 1.17 x. The correlation coefficient between y and x was 0.91 (P less than 0.001). The increment in height which will occur during 48 weeks of treatment can be predicted from the response to 10 days of treatment by this equation. The SE of the prediction averages +/- 1.2 cm/yr.