A series of 27 glioblastomas multiforme were studied to compare the prognostic value of age and histologic parameters known to influence survival with that of proliferating cell nuclear antigen (PCNA). The labelling index (LI) of PCNA has been found in previous studies to correlate well with flow cytometry, tritiated thymidine, bromodeoxyuridine incorporation and Ki 67. The histologic parameters estimated were endothelial proliferation, number of mitoses per high power field, cellularity and a grading of necroses. The correlation coefficients between survival and each of the histologic parameters as well as with age were found to be negative but statistically insignificant. The values of these correlations in descending order were survival/age (r = -0.22), survival/endothelial proliferation (r = -0.20), survival/mitoses (r = -0.16), survival/cellularity (r = -0.07) and survival/necroses (r = -0.04). The correlation coefficient between survival and PCNA LI was also found to be negative, higher than those between survival and other estimated parameters (r = -0.37) but in a wide variation and still statistically insignificant. Despite this variation, our finding that for PCNA LI > 35% no patient survived more than 6 months may be of clinical importance.