Recent clinical studies have shown that inorganic arsenic trioxide (As2O3) at low concentrations induces complete remission with minimal toxicity in patients with refractory acute promyelocytic leukemia (APL), Preclinical studies suggest that As2O3 induces apoptosis and possibly differentiation in APL cells. Like APL cells, neuroblastoma (NB) cells are thought to be arrested at an early stage of differentiation, and cells of highly malignant tumors fail to undergo spontaneous maturation. Both APL and NE cells can respond with differentiation to retinoic acid (RA) treatment in vitro and probably also in vivo. For that reason we investigated the effect of As2O3 alone and in combination with RA on NE cell lines. In vitro, the number of viable NE cells was reduced at As2O3 concentrations around 1 muM after 72 h exposure. The IC50 in six different cell lines treated for 3 days was in the 1.5 to 5 muM concentration interval, the most sensitive being SK-N-BE(2) cells derived from a chemotherapy resistant tumor. The combined treatment with RA (1 and 3 muM) showed no consistent additional effect with regard to induced cell death. The effect of As2O3 on NB cell number involved As2O3-induced apoptotic pathways (decreased expression of Bcl-2 and stimulation of caspase-3 activity) with no clear evidence of induced differentiation. The in vivo effect of As2O3 on NB growth was also investigated in nude mice bearing tumors of xenografted NE cells. Although tumor growth was reduced by As2O3 treatment, complete remission was not achieved at the concentrations tested. We suggest that As2O3, in combination with existing treatment modalities, might be a treatment approach for high risk NB patients.