Three cases of idiopathic myelofibrosis with partial trisomy of the long arm of chromosome 1 are described. Partial trisomy 1q was the only karyotypic change detectable in unstimulated peripheral blood cell cultures of one and bone-marrow cultures of two patients at diagnosis. The extra segment from chromosome 1 was located on different karyotype sites, i.e. 1qter, 1p34 and 6p22-23; 1q21-32 was the shortest overlapping region and the only trisomic segment in one of the three patients. These findings suggest that partial trisomy 1q is a primary chromosome aberration in myelofibrosis relevant in the pathogenesis of this hematologic disorder.