A morphometric evaluation of number and grouping of megakaryocytes (MK) in five different groups of chronic myeloproliferative disorders (CMPD) was performed by counting 60 high power fields equaling approximately 14.28 mm2 of haematopoiesis in each case. Twenty-one up to 29 cases were evaluated for each of five categories of CMPD and one control group; a total of 132 cases of CMPD and 33 control cases were used. The mean number of MK per square millimetre was 15.54 +/- 1.53 in chronic myeloid leukaemia of common or granulocytic type (CML.CT), 69.91 +/- 5.85 in CML with megakaryocytic increase (CML.MI), 59.59 +/- 3.27 in polycythaemia vera (P. vera), 59.85 +/- 4.59 in primary thrombocythaemia (PTH), 67.58 +/- 4.11 in chronic megakaryocytic granulocytic myelosis (CMGM), and 19.7 +/- 3.07 in controls. The distinction between free or isolated MK, and between clustered or grouped MK corresponds to the total cell counts of MK in the various groups of CMPD. Clustering of MK was significantly higher in CMGM and PTH compared to other groups, but the difference between them was not statistically significant. Significant differences in the mean number of MK were obtained between controls and CML.CT on the one hand and all other groups of CMPD on the other. The results further support the histological sub-classification of CMPD according to the primary disorders of the Hannover classification (not advanced by sclerosis, fibrosis or excess of blasts, respectively).