Of 8 children with GHD (growth hormone deficiency) on prolonged GH treatment, growth rates are comparable to those reported by the NCGS (National Collaborative Growth Study) for idiopathic GHD at 1 and 2 years. The mean change in height over a 28-month period for the treated group in our study was +1.8 S.D. while it was -0.25 S.D. in the untreated group (p<0.001). No known complications of GH treatment were encountered. Furthermore, because of an association between 18q deletion, hypomyelination and cognition, some non-statural benefits of GH treatment were examined: specifically, the performance intelligence quotient (pIQ) was measured using a detailed battery of neuropsychological instruments. The pIQ was measured because many of the children are hearing impaired and a full scale IQ, which relies on verbal skills, would underestimate their abilities. The GH-treated group was compared to an untreated group. The GH-treated group (n=8) showed an increase in pIQ of 23 points (range 0 to +47) while the untreated group (n=6) showed no change (range -2 to +6)(p=0.003). Based on these observations, we conclude that GHD children with 18q deletions respond favorably to GH therapy in terms of both linear growth and pIQ. In contrast, children with 18q deletions with abnormal growth who are not classically GH deficient, show little change in height S.D. and pIQ over time. Therefore, we propose to study (NEW STUDY) whether children with 18q deletions with abnormal growth can benefit from GH treatment. For purposes of this proposal, abnormal growth is defined as a growth velocity <-1 S.D.