Chromosome studies of five patients with myelofibrosis and myeloid metaplasia were carried out on bone marrow cells and/or on peripheral blood without PHA. Abnormal clones were found in three patients. Such clones were a minority, compared with the number of cells with normal karyotypes in all three patients. Chromosomes abnormalities consisted of 5q- (case 5), 13q- (case 2), and a small supernumerary acrocentric marker (case 3). One of our five patients, a woman aged 75 (case 1), showed a constitutional karyotype 46,XX,inv(5)( p15q11 ). The same chromosome rearrangement was present in 100% of the stimulated peripheral lymphocytes of this patient and in one of her sons with a normal phenotype. One patient (case 4) had a normal karyotype. These results are discussed and compared with data from the literature concerning myelofibrosis and other myeloproliferative diseases.