The relative effects of growth hormone (GH) on GH-deficient (GHD) children with and without severely delayed skeletal maturation prior to treatment are unclear. Methods: Pre-pubertal GHD children enrolled in the National Cooperative Growth Study were divided into two groups: severe pretreatment BA delay (BA Z-score ≤–2, group 1) and mild to moderate delay (BA Z-score between –2 and 0, group 2). BA Z-scores (BAZ), height Z-scores and predicted adult height (PAH) Z-scores at pubertal onset were determined after initiation of recombinant human GH (rhGH) therapy. Results: Height Z-scores increased from baseline to the start of puberty in both groups (group 1: mean difference of +0.99, males and +1.02, females; group 2: +0.68, males and +0.75, females). Mean BAZ also increased in all group 1 children (+1.62, males and +1.08, females) with no corresponding change in mean BAZ in group 2. PAH Z-scores increased in both groups. There was no evidence of undue advance in BA since mean BAZ remained well below zero in group 1 and was unchanged in group 2. Conclusion: rhGH has a beneficial effect on growth in prepubertal GHD children at all levels of pretreatment BA delay.