Growth hormone (GH) has been implicated in the pathogenesis of proliferative diabetic retinopathy. We sought to determine whether this could be mediated by an effect of GH on proliferation of endothelial cells, and, for this purpose, established long-term cultures of human retinal microvascular endothelial cells (hREC) from normal postmortem human eyes. High-purity (greater than 95%) hREC preparations were selected for experiments, based on immunofluorescence with acetylated low density lipoprotein (LDL) and anti-factor VIII-related antigen. Growth requirements for these cells were complex, including serum for maintenance at slow growth rates and additional mitogens for more rapid proliferation. Exposure of hREC to physiologic doses of human GH (hGH) resulted in 100% greater cell number vs. control (P less than 0.01) but could be elicited only in the presence of serum. When differing serum conditions were compared, hGH stimulated [3H]thymidine incorporation up to 1.6- to 2.2-fold under each condition and increased DNA content significantly in the presence of human, horse, and fetal calf serum. Depending on the culture conditions used, the threshold hGH concentration for significant stimulation of hREC proliferation was 0.4-4 micrograms/liter. In contrast, proliferation of human umbilical vein endothelial cells was not significantly altered by hGH added to concentrations as high as 200 micrograms/liter. In summary, hREC respond to physiologic concentrations of hGH in vitro with enhanced proliferation. This specific effect of GH on retinal microvascular endothelial cells supports the hypothesis of a role for GH in endothelial cell biology.