Pulmonary adenocarcinoma is the most common, and the most diverse form of primary lung carcinoma. The histological complexity of these tumours poses problems for pathologists. The current WHO classification of pulmonary adenocarcinoma does not adequately address a number of clinically relevant, biological factors. The accurate diagnosis of adenocarcinoma on small biopsy specimens, accounting for most diagnoses of this disease, is challenged by the absence of tumour architecture in most samples. Tumours showing a pure bronchioloalveolar (BAC) pattern are now best regarded as adenocarcinoma-in-situ; yet invasive adenocarcinomas may also show elements with the BAC pattern, dictating a better prognosis but biologically not necessarily in-situ disease. Multifocal BAC-pattern adenocarcinoma still poses considerable conceptual and diagnostic problems. In small tumours the papillary pattern, especially when micropapillary, confers a poor prognosis but this is not reflected in larger tumours. In early tumours of predominantly BAC (in-situ) pattern, the identification of invasion is particularly difficult, yet minor degrees of infiltration seem not to degrade prognosis. It may therefore be possible to define a minimally invasive category of adenocarcinoma. Consequently, there are a number of issues to consider when reporting this tumour type, depending on the nature of the diagnostic specimen. The rapid emergence of chemotherapeutic agents with histology-specific efficacy will increase the need for more accurate and specific diagnosis of adenocarcinoma on small samples. Immunohistochemistry may help suggest this diagnosis when the features are non-specific but immunohistochemical findings are not diagnostic of this form of lung cancer. The emerging clinical and prognostic relevance of a number of histological features in these complex tumours strengthens the argument in favour of including quantitative detail of pattern sub-types in reports on resected tumours.