The peripheral blood WBC size distribution was assessed by morphological and volume measurements in 73 patients with B-cell chronic lymphocytic leukaemia (CLL) and prolymphocytic leukaemia (PLL). Patients with typical CLL, with less than or equal to 10% prolymphocytes, had a homogeneous major population of small cells which could be recognized both by morphology and volume (median volume 211.5 +/- 32.5 fl). In PLL, the volume of the main cell component was significantly larger than in CLL: in two-thirds of cases the major cell population was distributed in a first lognormal fitted curve of the volume histogram, with median volume 281.8 +/- 38.0 fl; in the remaining cases the main cell component showed a larger median volume (353.5 +/- 71.9 fl) contained within the second lognormal curve which was preceded by a minor peak. CLL patients with 11-55% prolymphocytes (CLL/PL) had characteristic cell volume histograms in which two lognormal curves could always be fitted: in 80% of cases the main cell component was located in a first curve with median volume of 257.9 +/- 28.6 fl; in the remaining cases the major population was represented by cells with median volume of 349.0 +/- 83.9 fl distributed in a second peak. Although in both CLL and CLL/PL the majority of cells was defined morphologically as small, the median volume of these lymphocytes was significantly larger in CLL/PL. The degree of concordance in the assessment of cell size between morphology and volume measurements was high in CLL, whereas in CLL/PL and PLL morphology underestimated the cell size of the major population, compared with its actual volume, in over 50% of cases. We conclude that the identification of prolymphocytes as larger cells in blood films may be hampered by distortions and artefacts of spreading. Volume measurements may provide a more objective indicator of the cell populations in this group of disorders.