The criteria for the separation of invasive lobular and ductal carcinomas are analysed. Lobular tumours account for 14% of invasive cancers in our material. The widely differing figures given in the literature are mainly attributable to intrinsic difficulties of classification. In addition to the classical dissociated-cell patterns with single filing, a targetoid appearance and related features, variants with trabecular, loose alveolar and tubular features are recognized; a ‘solid’ variant requires further investigation. The variants either represent better differentiated forms, or in some cases, an ‘earlier’ phase in the production of the more traditional Indian file formations and dartboard patterns. Focal signet-ring cell differentiation is another newly-recognized feature. The theoretical and practical implications of these variants are considered. The division into invasive lobular and ductal carcinomas is not as easy as most of the literature implies. Five per cent of cases could not be so classified and, in some of these unclassified cases, both ductal and lobular differentiation may be present. In the diagnosis of problem cases, no single parameter proved reliable but a combination of several parameters enables one to make an objective diagnosis in about 95% of cases.