BACKGROUND The purpose of the current study was to determine the accuracy of the cytologic diagnosis of bronchioloalveolar carcinoma (BAC) by fine-needle aspiration biopsy (FNAB). METHODS During a 4-year period (1994-1998), 1664 lung FNABs were performed. Forty-nine patients with BAC diagnosed by FNAB and/or surgical biopsy formed the basis of this study. RESULTS Twenty-four patients diagnosed with BAC by FNAB had histologic confirmation. Surgical pathology revealed BAC in 15 patients with a cytologic diagnosis of large cell carcinoma (LCA) or adenocarcinoma (ACA). Nine patients diagnosed with BAC by FNAB were found to have ACA histologically. One unsatisfactory aspirate was diagnosed as BAC by surgical pathology. Review of 15 FNAB specimens with a diagnosis of LCA or ACA revealed cytologic features typical of BAC. In six aspirates, additional features such as pronounced nuclear crowding and overlapping, variation in nuclear size, and increased number of pleomorphic cells interfered with the FNAB diagnosis of BAC. Nine FNABs with a diagnosis of BAC were found histologically to have ACA with a focal BAC growth pattern. One unsatisfactory FNAB aspirate diagnosed as BAC histologically was due to sampling error. CONCLUSIONS A diagnosis of BAC by FNAB is possible using conventional cytologic criteria. Some BACs show pronounced nuclear crowding and overlapping, variation in nuclear size, and an increased number of pleomorphic cells cytologically, which may interfere with an FNAB diagnosis of BAC. FNABs from ACA cases with a focal BAC pattern remain a diagnostic dilemma due to the nature of the lesion. In addition, sampling error by FNAB can be a diagnostic pitfall. Cancer (Cancer Cytopathol)