The morphology of the cells from 29 cases of T-prolymphocytic leukaemia (T-PLL) was studied by light (LM) and transmission electron microscopy (TEM) and was compared with that of 33 B-cell PLL. The membrane phenotype of T-PLL cells was T4+, T8- in two-thirds of the cases, others being T4- T8+ or T4+ T8+. Two morphological types of T-PLL were defined according to the nuclear features: regular (55% of cases) and irregular (45% of cases). T-PLL cells with a regular, round or oval, nuclear outline resembled B-PLL cells but had less abundant cytoplasm and a higher nucleo-cytoplasmic ratio. Irregular T-prolymphocytes displayed a distinct convoluted nucleus. A 'small-cell' variant of T-PLL was recognized by TEM in six cases in which the diagnosis was uncertain by LM. A characteristic of all types of T-prolymphocytes by LM was the presence of a deep basophilic cytoplasm which by TEM corresponded to clusters of ribosomes and endoplasmic reticulum. No differences in clinico-haematological features or membrane markers were apparent between the morphological types of T-PLL, although it was noted that the three T4- T8+ cases had irregular cells and four of the small cell variant were T3- T4+. TEM permits a more precise assessment of the cytoplasmic organelles and nucleolus than LM analysis and facilitates the distinction between T-PLL and other leukaemias with a mature T-cell phenotype, namely adult T-cell leukaemia/lymphoma. Sezary syndrome and T-chronic lymphocytic leukaemia.