Thirty-five patients, 24 males and 11 females, with myelodysplastic syndromes were studied for the expression of c-myc encoded p67 oncoprotein. According to FAB classification, 5 patients had refractory anaemia (RA). 5 refractory anaemia with ringed sideroblasts (RARS), 17 refractory anaemia with excess of blasts (RAEB), 4 refractory anaemia with excess of blasts in transformation (RAEB-t), and 4 chronic myelomonocytic leukaemia (CMML). The mouse anti-human 9E10 derived monoclonal antibody in the standard APAAP technique for immunohistochemical analysis was used. A scoring method similar to that routinely used for endogenous neutrophil alkaline phosphatase estimation, was applied to obtain parametrically comparable results. In all but two MDS patients, the observed c-myc oncoprotein score values did not differ statistically from those found in the controls. The values did not correlate with peripheral blood or bone marrow blast cell numbers. In two out of 17 patients with RAEB in whom very high c-myc score values were found, acute non-lymphocytic leukaemia (ANLL) was diagnosed 30 and 45 days later, respectively. Furthermore, we found high c-myc score values in three patients with RAEB and in two patients with RAEB-t during the ANLL phase which was diagnosed six to ten months after the initial study. Our data suggest that c-myc activation may be seen in all cases of MDS in overt ANLL phase, and also in some RAEB patients a few weeks before diagnosis of overt ANLL. The possible prognostic value of c-myc activation in MDS patients remains to be clarified.