To assess agreement between cyto-histological results and fine needle aspiration (FNA) biopsy of malignant pulmonary lesions, and to study the relation with type of lesion. specimen and impact on patient management. Retrospective study of FNA performed over the past 4 years if a biopsy was available (obtained by fiberoptic bronchoscopy, thoracotomy or biopsy of extrapulmonary organs) for comparison. We recorded overall agreement (OA) and agreement by type of disease or neoplasm (DA). Also studied were the features of the lesion, the puncture technique and material obtained in function of agreement. Eighty samples were available for comparison. OA was 58.7% (K = 0.17). DA was good for epidermoid carcinoma (87%, K = 0.64) and poor for adenocarcinoma (87.5%, K = 0.30). The lowest agreement was for undifferentiated large cell carcinoma (10.3%, K = 0.07). In such cases FNA specimens were not useful for classifying 61.5% of adenocarcinomas and 21.6% of epidermoid carcinomas. Cyto-histological inaccuracy was clinically significant, however, in only 3 (3.7%) patients. Lesions for which diagnosis was consistent were larger in size (4.6 +/- 2.2 versus 4 +/- 1.6 cm, p = NS), were nearer to the visceral pleura (1.5 +/- 2.3 versus 2 +/- 2.2 cm, p = NS) and tended to have been sampled with the guidance of computerized tomography (65% versus 35%), although this did not affect the features or amount of material obtained. We found poor OA for adenocarcinoma and undifferentiated large cell carcinoma. Although disagreement was clinically significant in only 3.7% of cases, the implications indicate that the specificity of the technique should be improved, above all in small cell carcinomas. We observed no differences as to type of lesion or specimen obtained that might predict interpretive difficulties.