Theileria is an intracellular parasite that causes lymphoproliferative disorders in cattle, and infection of leucocytes induces a transformed phenotype similar to tumour cells, but the mechanisms by which the parasite induces this phenotype are not understood. Here, we show that infected B lymphocytes display constitutive phosphoinositide 3-kinase (PI3-K) activity, which appears to be necessary for proliferation, but not survival. Importantly, we demonstrate that one mechanism by which PI3-K mediates the proliferation of infected B lymphocytes is through the induction of a granulocyte-monocyte colony-stimulating factor (GM-CSF)-dependent autocrine loop. PI3-K induction of GM-CSF appears to be at the transcriptional level and, consistently, we demonstrate that PI3-K is also involved in the constitutive induction of AP-1 and NF-kappaB, which characterizes Theileria-infected leucocytes. Taken together, our results highlight a novel strategy exploited by the intracellular parasite Theileria to induce continued proliferation of its host leucocyte.