Vitamin D has been shown to inhibit growth of human retinoblastoma in tissue culture and nude mouse heterografts. We have described a heritable transgenic mouse model of retinoblastoma. The in vivo efficacy of 1,25-dihydroxycholecalciferol (vitamin D3) was examined by administering this agent to transgenic mice with retinoblastoma. Forty-six 8-10-week-old transgene-bearing mice were injected intraperitoneally for 5 wk. Experimental animals received 0.05 microgram (15 animals) or 0.025 microgram (15 animals) of vitamin D. Sixteen control animals received only a mineral oil vehicle. Eyes were enucleated at 5 mo and were examined histologically by two investigators in a masked fashion. All control animals demonstrated bilateral involvement of retinoblastoma. Four eyes in the low-dose group and six eyes in the high-dose group had no evidence of retinoblastoma. Eyes treated with vitamin D3 showed less extensive involvement of the retina by retinoblastoma. Vitamin D-treated animals demonstrated tumors confined to the retina, whereas control animals demonstrated larger tumors, more often invading the vitreous, anterior chamber, and choroid. Thus, Vitamin D inhibited the growth and local extension in a dose-dependent fashion.