Medulloblastomas (MBs) are the most common malignant brain tumors of childhood. Antitumor agents promoting long-term survival with limited toxicities are thus far lacking. Preliminary findings suggest that retinoic acid (RA) derivatives (retinoids) exert antitumor effects by inhibiting cell proliferation and inducing cell differentiation, apoptosis, and growth arrest, and RAs have been specifically shown to induce apoptosis in some MB cells. However, there is no conclusive evidence of retinoids inducing cell growth arrest in MBs. The aim of this study is to investigate whether retinoids play a role in cell-cycle arrest of MB cells. All-trans-retinoic acid (ATRA) was selected for these studies as it is known to have the capacity of inducing cell cycle arrest and apoptosis in other types of cancer cells. Three MB cell lines (DAOY, D283 and D341) were subjected to ATRA treatment. The proportions of cells in the G0/G1 phase of cell cycle and in apoptosis were evaluated. The results showed that cell growth arrest, rather than apoptosis, was the main mechanism by which RA inhibited cell proliferation in the MB cell line DAOY, but not in the others (D283 and D341). Decreased expression of CyclinD1 and C-myc which regulate the transition of cell cycle was observed in DAOY cells following drug treatment, suggesting that these genes might be involved in ATRA retardation of cell cycle progression. Expression of RARbeta, a mediator of the action of retinoids, was also induced by RA in DAOY cells, implying that RAR-beta might also be involved in the mechanism of RA-induced cell cycle arrest. In conclusion, we have provided evidence for the first time that RA may induce cell cycle arrest in vitro in DAOY MB cells via inhibition of CyclinD1 or C-myc.