Two brothers with familial isolated growth hormone deficiency type IA homozygous for the same 6.7 kb deletion on chromosome 17 including the growth hormone gene were intermittently treated with various forms of hGH for more than 7 years. While the elder brother (Patient 1) showed a good growth response to pituitary hGH, the younger one (Patient 2) developed high titre growth blocking hGH antibodies early in the course of treatment and grew only 2.2–3.9 cm/year on a hGH dose of 12–26 IU/m2 per week. When the younger brother was changed to a higher dose (33 IU/m2 per week) of biosynthetic methionyl hGH he had striking catch-up growth and he has subsequently maintained a height velocity of 10.0 cm/year for the last 2 years. During this time his antibody titres have decreased over 1000-fold. These findings demonstrate that therapy with biosynthetic methionyl hGH may provide an effective form of treatment for subjects with isolated growth hormone deficiency type IA who do not grow in response to native hGH, and imply that biosynthetic methionyl hGH may be less antigenic than pituitary derived hGH.