Abstract A number of biological and predictive markers of non-small cell lung cancer (NSCLC) have been sought, but these have so far been mainly evaluated on surgically resected specimens. Given that fine needle aspiration biopsy (FNAB) is being increasingly used in the diagnosis of NSCLC, its application could be extended to the immunocytochemical detection of biological parameters at the time of diagnosis before surgery. In order to assess the reliability of estimating biological markers on fine needle aspirates (FNAs) from NSCLC, the aim of this study was to compare Ki67 growth fraction, p53 and bcl-2 protein expression as revealed by the immuncytochemical assessement of FNAs obtained from surgical samples with the immunohistochemical results obtained from the corresponding histological sections. FNAs were performed on surgical specimens obtained from 29 NSCLC patients. Ki67, p53 and bcl-2 were cytologically and histologically evaluable in respectively 25, 27 and 19 cases. Concordance between FNAs and corresponding paraffin sections was 84% for Ki67, 93% for p53 and 95% for bcl-2. All of the specimens whose biological parameters were studied by immunocytohistochemistry also underwent flow cytometric DNA analysis of FNAs taken from fresh surgical specimens. Of the 29 cases, 22 were aneuploid and seven diploid. The S-phase fraction (SPF) was evaluable in 62% of cases. Comparison of SPF results on FNAs with Ki67 values evaluated on the corresponding histologic and cytologic specimens, revealed a significant correlation only with histology. Good reproducibility was also found in relation to the immunocytochemical results obtained on FNAs from different areas of the same tumour, showing that tumour heterogeneity does not affect the method. The concordance between the immunocytochemical and immunohistochemical results suggests that FNAB may be a reliable procedure for the biological characterization of NSCLC. Given its limited invasiveness, FNAB could be used in vivo for the preoperative assessement of biological parameters in patients with operable or metastatic NSCLC.