Bone marrow chromosomes were studied in a series of 50 consecutive, unselected cases of untreated polycythaemia vera (PV). A pathological cell line was present in 7 patients (= 14%). 3 patients each had 1 extra chromosome no. 8 and 1 extra no. 9. 2 patients had a similar but not identical marker chromosome including material from the long arms of chromosome no. 1. 1 of these patients also had a 20q- deletion. 1 patient had an extra unidentified isochromosome. 1 patient had a monosomy 16. In 5 cases the abnormal clone dominated over cells with normal karyotype. In each of 6 patients a singular hyperdiploid cell was observed. 1 or several polyploid or randomly hypodiploid cells were found in most patients. The findings in our material are compared to other reports of chromosome studies in PV and allied disorders. Except for the 20q- deletion no abnormality typical for PV has so far been detected, but some chromosomes seem to be involved more often than others (1, 8, 9, 20). Patients with chromosome aberrations did not show any clinical, laboratory or morphologic features which could separate them from cases with normal karyotypes. The prognostic significance of the chromosome abnormalities encountered in early PV cannot be defined at present.