Aneuploidy and chromosomal aberrations are hallmarks of most human epithelial malignancies. Here we show that overexpression of the oncogenic kinase Pim-1 in human prostate epithelial cells induces genomic instability by subverting the mitotic spindle checkpoint. Cells overexpressing Pim-1 have a defect in the mitotic spindle checkpoint, abnormal mitotic spindles, centrosome amplification, and chromosome missegregation. Polyploidy and aneuploidy ensue due to a delay in completing cytokinesis. These results define a novel role for elevated Pim-1 expression in promoting genomic instability in human prostate tumors.