Six cases of Burkitt's lymphoma (BL) and 12 cases of Burkitt's-like lymphoma (BLL) were classified by using strict histologic and cytologic criteria. These cases were processed for electron microscopy and analyzed by using computerized image analysis techniques. Form factor (4 pi A/P2) was used to measure nuclear contour irregularity. The mean of the standard deviation (SD) of nuclear area and form factor was used to assess pleomorphism. Overall, there were 8 similarities and 10 statistically significant dissimilarities out of 18 parameters analyzed. The similarities (p greater than 0.05) between the BL and BLL groups included the means of form factor, nuclear area/cytoplasmic area, SD of nuclear area/cytoplasmic area, number of nuclear pockets/100 nuclei, percentage of cells with nuclear pockets, number of lipid droplets/micron 2 of cytoplasm, percentage of cells with lipid droplets and number of mitochondria/micron 2 of cytoplasm. The dissimilarities (p less than 0.05) included the means of nuclear perimeter, SD of nuclear perimeter, nuclear area, SD of nuclear area, cellular area, SD of cellular area, cytoplasmic area, SD of cytoplasmic area, SD of form factor, and nucleolar frequency. Multiparameter analysis clearly separated these 18 patients into two distinct groups and confirms that the subtleties used in the histologic classification of these lymphoma subtypes are meaningful. Sixteen cases of BL and BLL were snap-frozen in isopentane and analyzed by using 16 lymphoid surface markers. All of the immunoglobulin-positive BL were of the mu isotype, whereas the BLL cases were divided between mu (6 cases) and gamma expression (4 cases). All 4 of the BL evaluated manifested CALLA expression, whereas 3 of the 11 BLL evaluated coexpressed CALLA. One BL case was of a pre-pre-B phenotype and one BLL case was of pre-B phenotype. The BL and BLL were compared to 49 cases of SIg (+) large cell lymphomas. The high incidence of coexpression of lambda, CALLA, and Ki-67 in BL and BLL separates these lymphomas, as a group, from the large cell lymphomas. We have also determined from this study that the separation of patients into distinct BL and BLL subtypes is clinically relevant. The BL group were all children (median of 6.5 years) compared with the BLL group who were all adults (median of 63 years). The complete remission rate was higher in the BL (67%) than in the BLL group (25%).4off