The Mxi1 protein negatively regulates Myc oncoprotein activity and thus potentially serves a tumour suppressor function. MXI1 maps to chromosome 10q24–q25, a region that is deleted in some cases of prostate cancer. We have detected mutations in the retained MXI1 alleles in four primary prostate tumours with 10q24–q25 deletions. Two tumours contained inactivating mutations, whereas two others contained the identical missense mutation. Fluorescence in situ hybridization also demonstrated loss of one MXI1 allele in an additional tumour lacking chromosome 10 abnormalities. MXI1 thus displays allelic loss and mutation in some cases of prostate cancer that may contribute to the pathogenesis or neoplastic evolution of this common malignancy.