With the availability of an easily produced and safely administered form of recombinant human growth hormone (GH), the clinical indications for its use have increased. Recent studies have shown that GH therapy is both safe and effective in the treatment of growth failure in children with chronic renal insufficiency. Similarly, GH has been used to treat growth failure in children after renal transplantation. GH therapy increases growth in these patients, but in some of them increased organ rejection or worsening of kidney function occurs. GH is a neuroendocrine peptide that is important to somatic growth, but it has also been shown to have pleiotropic effects on many cells and organ systems including the immune system. We have shown that GH in vitro augments the responses during a mixed leukocyte culture in normal adult volunteers. GH also augments the responses to donor-specific alloantigens in some patients. We conclude that if in vitro assays such as these can successfully identify those patients who are at risk for organ rejection and worsening kidney function, then GH therapy is safe in children with growth failure after renal transplantation.