The introduction of the laser scanning cytometer offers new capabilities in cell proliferation research, through its capacity for validation of each and every cell event through direct visualization on the microscope slide. In this study, we report a direct comparison of proliferation data derived from flow and laser scanning cytometry of human tumor nuclei labeled in vivo with bromodeoxyuridine (BrdUrd). Nuclear suspensions from 19 invasive ductal breast carcinomas and 12 gastric adenocarcinomas were prepared and analysed for BrdUrd uptake and DNA content. Specimens were analysed using a FACScan and then prepared on cytocentrifuge preparations for laser scanning cytometry. DNA index, labeling index (LI), duration of S-phase (Ts) and potential doubling time (Tpot) were calculated using standard procedures. There was an excellent correlation between the two techniques in the calculation of DNA index (R = 0.983, P > 0.0001) and LI (R = 0.924, P > 0.0001). The Ts proved more problematical (R = 0.448, P = 0.0115) but the Tpot showed closer agreement (R = 0.851, P > 0.0001) as the LI was the dominant determinant of Tpot. No single parameter could be identified as the major source of variation between the two techniques. We conclude that the laser scanning cytometer produces data equivalent to that obtained by flow cytometry.