Loss of function of pins (partner of inscuteable) partially disrupts neuroblast (NB) polarity and asymmetric division, results in fewer and smaller NBs and inhibits Drosophila larval brain growth. Food deprivation also inhibits growth. However, we find that the combination of loss of function of pins and dietary restriction results in loss of NB asymmetry, overproliferation of Miranda-expressing cells, brain overgrowth and increased frequency of tumour growth on allograft transplantation. The same effects are observed in well-fed pins larvae that are mutant for pi3k (phosphatidylinositol 3-kinase) or exposed to the TOR inhibitor rapamycin. Thus, pathways that are sensitive to food deprivation and dependent on PI3K and TOR are essential to suppress tumour growth in Drosophila larval brains with compromised pins function. These results highlight an unexpected crosstalk whereby the normally growth-promoting, nutrient-sensing PI3K/TOR pathway suppresses tumour formation in neural stem cells with compromised cell polarity.