Aqueous humor from 30 enucleated retinoblastoma-bearing eyes and 9 cell cultures of retinoblastoma were tested for angiogenesis activity using capillary endothelial cell migration assays. Aqueous from 90% of retinoblastoma eyes stimulated endothelial cell migration, compared with 25% of control eyes. Five of nine retinoblastoma cultures stimulated endothelial cell migration and five showed chorioallantoic membrane (CAM) neovascularization when compared with positive controls for angiogenesis activity. These results suggest an inverse relation between the length of time in culture and the ability of these cells to stimulate angiogenesis using these assays. Cells in culture for short periods of time gave positive test results, whereas continuous cell lines of retinoblastoma were consistently negative. While aqueous humor from retinoblastoma-bearing eyes demonstrated significant angiogenesis activity, cultured retinoblastoma cells show this only in early passages. Several possible explanations for this are considered.