Recent molecular biological investigations have shown that genetic information on the short arm of chromosome 17 is frequently deleted or inactivated in acute myeloid leukaemias and a number of solid tumours. In earlier studies we have found clonal occurrence of a 17p deletion (17p-) in patients with refractory anaemia. In the present study we investigated the time patterns of 17p- and morphological characters of bone marrow cells in 14 myelodysplastic patients with clonal 17p- who have been reinveststigated within 3- to 6-month intervals for 15–56 (median 37) months. We found that the 17p- frequency increases significantly with time. Further, the deletion is positively correlated with bone marrow myeloblast frequencies and negatively with those of maturing granulocytes (myelocytes and later forms). These results suggest that 17p- interferes with and delays granulocyte maturation. Inactivation or deletion of genetic material in 17p in myelodysplastic syndrome, acute myeloid leukaemia, and a wide variety of solid tumours further suggests that genes present on 17p may have a central position in malignant transformation of cells in a number of different tissues.