Proliferative rate and DNA ploidy in 23 squamous cell carcinoma of the lung (SCCL), 10 adenocarcinomas (ACL) and 4 metastatic tumours (MT) were studied. The proliferative potential of the tumours was assessed with the use of three methods: bromodeoxyuridine labelling index (BrdUrd LI), S-phase fraction (SPF) and argyrophilic organizer regions (AgNOR) technique. Differences in the proliferating rate between different histological groups of tumours and within groups were shown by the BrdUrdLI and AgNOR technique. However, for the BrdUrdLI only these differences were statistically significant. No correlation was found between the values for all examined tests and the clinical stage of the tumours. The broadest range of the BrdUrdLI values was for SCCL and the mean BrdUrdLI was highest in this histological group. The number of SPF cells in metastatic tumours was lower than that in two other histological groups, however it was not a significant difference. The highest number of AgNORs and tot AgNOR area per cell was noticed again in SCCL, however, these values were not significantly different (p = 0.189) from other histological groups. Also the mean size of AgNOR particle area did not differ between groups (p = 0.279). Twenty six of thirty seven (70%) tumours were aneuploid. The highest percentage was recorded for SCCL (81%), while the lowest (25%) for MT. In the aneuploid tumours no significantly higher proliferation rate was observed as assessed by the BrdUrdLI and SPF. However, higher AgNOR numbers (p = 0.0472) and higher tot AgNOR area per cell (p = 0.0128) in aneuploid than in diploid tumours were observed. There was no correlation between BrdUrdLI, SPF and AgNOR counts.