A 72 year old man was diagnosed with chronic myelomonocytic leukaemia (CMML) according to the FAB group classification. He presented with symptoms of anaemia, urinary frequency, hesitancy, and nocturia. He was later admitted with acute urinary retention and acute renal failure, which resolved with treatment. A transurethral resection of the prostate was performed. Histological examination showed fibromuscular hyperplasia with dense infiltration by myelomonocytes which stained positively with chloroacetate esterase; immunohistochemical staining was positive for lysozyme, CD43, CD45, and CD68. Following treatment with oral etoposide he transformed to acute myeloid leukaemia and eventually died. Myelomonocytic infiltration of the prostate has not been reported before. This case extends the spectrum of disease previously recognised in CMML.