Bronchioloalveolar carcinoma (BAC) is a form of peripheral lung adenocarcinoma growing as a single layer of malignant cells along the walls of terminal airways. The existence of BAC as a separate clinico-pathological entity has been a matter of controversy, mainly because its histogenesis is uncertain and it is not easily distinguishable from conventional lung adenocarcinoma (CLA). Three subtypes of BAC have been described using histological and cytological criteria: mucinous, non-mucinous, and sclerosing. The clinical behaviour of BAC appears to be dependent on the histological subtype. The different morphological patterns and clinical outcome of the subtypes of BAC suggest that their biological behaviour may be different from one another and from CLA. This study has investigated 58 BACs (10 mucinous, 40 non-mucinous, and 8 sclerosing) and 50 control CLAs for mutations at codon 12 of the K-ras oncogene. Twenty-one (36 per cent) BACs and 13 (26 per cent) CLAs showed K-ras mutations. A clear association (P < 0.0001) between K-ras mutations and the mucinous type of BAC was observed: all 10 mucinous tumours examined were scored positive for mutations in the K-ras gene, while only 9 (23 per cent) of the 40 non-mucinous and 2 (25 per cent) of the 8 sclerosing BACs were found to be positive. The frequency of ras mutations in non-mucinous BAC, sclerosing BAC, and CLA was not statistically different. Our data indicate that BACs are a heterogeneous group of lung tumours and that the mucinous form might represent a biological entity separate from both the other two BAC types and CLA.