Insulin-like growth factor-I (IGF-I) was found to promote proliferation, cell survival, and inhibition of apoptosis. But in some instances, IGF-I was found to mildly induce apoptosis, i. e. Fas-mediated apoptosis in human MG63 osteosarcoma cells. In the present study, we intended to further investigate IGF-I dependent pathways leading either to proliferation and cell survival or to cell death. MG63 osteosarcoma cells were treated with serum free medium alone or in combination with IGF-I, a neutralizing antibody against the human IGF-I receptor (alphaIR-3) or non-immune control IgG (1) for two to six days. We investigated cell survival (cell count), proliferation (CD71-FACS), apoptosis (Annexin-V-FACS, Caspase-3 activity, PCD) and anti-apoptosis (112-Ser Bad phosphorylation), and regulation of IGF-I receptor surface expression (IGF-I receptor-FACS). We found that IGF-I treatment (48 h) stimulated cell growth and proliferation, but also mildly induced apoptosis. IGF-I activated specific apoptotic pathways (Caspase-3 activation, Annexin-V binding and DNA degradation), as well as anti-apoptotic signals (Bad phosphorylation at serine 112). alphaIR-3 blocked cell proliferation, strongly induced apoptosis, and inhibited Bad-phosphorylation. Thus, IGF-I treatment overall resulted in increased tumour cell mass, despite a detectable stimulation of apoptosis; in other words proliferation exceeded cell death. If IGF-I was first added on day 0, 2, or 4 of serum free culture, we found decreasing IGF-I specific effects on proliferation and apoptosis. In parallel, we found a down-regulation of IGF-I receptors (FACS) by serum withdrawal, which was partly reversed if IGF-I was added. Therefore receptor number might have an impact on IGF-I function in MG63 cells. In conclusion, co-activation of apoptosis and proliferation by IGF-I might result in higher cell turnover in MG63 osteosarcoma cells. Furthermore, in sarcomas or carcinomas showing clinical association to IGF-I levels and malignancy, IGF-I dependent apoptosis and proliferation could be a significant mechanism of malignant tumour growth.