We report a case of idiopathic myelofibrosis with trisomy 13 as the sole clonal aberration, as demonstrated by metaphase cytogenetics. The clinical course was especially poor in this case, with death in blast crisis occurring within two weeks from diagnosis. The dismal outcome bears striking similarity to two previous cases of idiopathic myelofibrosis and trisomy 13 reported in the literature. Therefore trisomy 13 may be a predictor of a rapidly fatal outcome in this otherwise indolent disease. Fluorescence in situ hybridisation (FISH) with a chromosome 13 specific probe may enhance the detection of this aberration, since only 50% of cases of idiopathic myelofibrosis are karyotyped successfully using conventional techniques.