Low bone mass and an increased prevalence of skeletal fractures are evident in the alcoholic population. Histomorphometric analysis of skeletal tissue from alcoholic patients reveals reduced osteoblast number and suppressed bone formation activity, with relative sparing of resorptive indexes. The decreased number of osteoblasts observed in alcoholic subjects results from either impaired proliferation or accelerated senescence. Polyamines and ornithine decarboxylase (ODC), the rate-limiting enzyme for polyamine synthesis, are essential for cell proliferation in a variety of cell types. To determine whether the consequences of ethanol on osteoblast number involve the modulation of polyamine biosynthesis, we examined the effect of ethanol on parameters of cell growth and ODC activity in a rat osteoblast-like osteosarcoma cell line (UMR 106-01). Ethanol markedly impaired DNA synthesis and cell proliferation in a dose-dependent fashion. Difluoromethylornithine, a specific inhibitor of ODC activity, induced a similar inhibition of UMR 106-01 cell proliferation, indicating the importance of the polyamine pathway in this osteoblastic cell line. Induction of ODC activity was impaired in ethanol-exposed cell cultures in a dose-dependent fashion that paralleled the antiproliferative effects. Finally, supplemental polyamine administration substantially improved DNA synthesis in ethanol-exposed UMR 106-01 cell cultures. These data confirm a direct inhibitory effect of ethanol on osteoblast proliferation that may in part explain the reduced bone mass observed in subjects who consume excessive amounts of alcohol. These findings also suggest that altered polyamine metabolism may be an important mechanism responsible for the antiproliferative effects of ethanol on the osteoblast.