A series of cases of high grade non-Hodgkin's lymphomas has been studied by morphology (110 cases), immunocytochemistry (90 cases), using reagents reactive in fixed paraffin-embedded tissue, and flow cytometry (77 cases). B-cell tumours constituted 67.0 per cent of the total, T-cell tumours 22.0 per cent, and unclassified cases 8.8 per cent. Immunocytochemistry revealed two anaplastic carcinomas. Of the 77 cases studied by flow cytometry, 67.5 per cent were diploid and 32.5 per cent DNA aneuploid. T-cell tumours were more likely to be diploid than B-cell tumours, though the difference did not reach statistical significance. T-cell tumours had a significantly lower proliferative index (%S + G2) than B-cell tumours (P = 0.002). The overall remission induction rate was 68 per cent and actuarial 3-year survival 47 per cent. There was a trend for cases with %S + G2 less than 22 per cent to survive longer (P = 0.07). This trend became statistically significant when aneuploid cases were added to the high PI group (P = 0.04). No correlation was seen between morphological or immunophenotypic groups and remission induction rates or survival.