Ploidy and growth fraction were analyzed by means of a computer-assisted image processor in surgically resected non-small cell lung cancer (NSCLC). This study was done in order (a) to evaluate the distribution of anti-Ki-67 immunostaining and (b) to correlate this distribution to ploidy status and pTNM stage of NSCLC. Thirty-two patients underwent a surgical resection for primary NSCLC following complete staging. Indirect immunoperoxidase reactions of monoclonal antibody Ki-67 were done on frozen tissue sections. Integrated optical density and index of stained nuclear surface were calculated by means of a computer-assisted image processor in 120 fields of each preparation in order to quantify the Ki-67 immunostaining. DNA content was determined by means of cytometry of Feulgen-stained cytological prints. The ploidy status was defined for each tumor by DNA index, percentage of hypodiploid cells, and type of DNA content histogram (near diploid, hyperdiploid, hypodiploid, and multiploid). Reproducibility of immunostaining quantitative analysis was demonstrated by iterative measurements of the same slide. Intratumoral heterogeneity of Ki-67 immunostaining induced integrated optical density variation assessed on six nonconsecutive tissue sections from at least two regions of the same tumor. This intratumoral variability was 15 times lower than integrated optical density variability between tumors. The Ki-67 immunostaining varied significantly according to the DNA content histogram type (P less than 0.05, Kruskal-Wallis test); most of the specimens with high Ki-67 immunostaining were multiploid or hypodiploid. Moreover, Ki-67 immunostaining correlated to the percentage of hypodiploid cells. Ki-67 immunostaining and ploidy status did not vary significantly according to the tumor-nodes-metastasis stage. We conclude that (a) quantitative analysis of Ki-67 immunostaining is a reliable evaluation of growth fraction in NSCLC if a large number of fields are analyzed to take into account intratumoral variability, (b) hypodiploidy and multiploidy are frequent abnormalities of DNA content, (c) Ki-67 immunostaining is significantly higher in hypodiploid and multiploid tumors. Thus, determination of growth fraction and ploidy in surgically resected NSCLC specimens may be considered as complementary prognostic parameters independent of the stage of the disease.