Proliferating cell nuclear antigen (PCNA) is a 36 kD nuclear protein involved in DNA replication that is believed to provide an indication of proliferation in some neoplasms. This study analyzes PCNA expression in 24 cases of primary non-small cell lung cancer using monoclonal PC-10 antibodies in paraffin embedded material. We found significant inter- and intra-tumoral variations in PCNA expression, and no statistically significant relation between the amount of PCNA expression and the size and location of tumors, index of mitosis, histological tumor type or patient age. We found a statistically significant relation (r = 0.47; p < 0.05) between survival and amount of PCNA expression in a sample of 19 cases, but no statistically significant differences in survival related to whether PCNA expression was slight (0-25), moderate (25%-50%) or high (> 50%), and no prognostic value for degree of PCNA expression.