Some controversy remains about the clinical or pathological definition of the different types of inflammatory breast cancer (IBC) and especially the diagnostic and prognostic value of dermal lymphatic involvement. Our purpose was to classify the different types of IBC for which diagnosis was confirmed intraoperatively and ascertain features allowing reliable diagnosis. We studied clinical findings, biological data, and treatment outcome in a series of 144 successive patients. Our results suggest that there are 2 biologically different entities i.e., true IBC and pseudo-IBC. True IBC, whose course is currently fatal in all cases, can be divided into 2 sub-categories i.e., common true IBC (75.7% of cases), in which inflammatory signs occur primarily or secondarily, and occult true IBC (13.2% of cases). Dermal emboli have been observed in 61% of common true IBC, but their absence did not alter the rapidly unfavourable outcome. Extensive lymph-node involvement, other biological features and survival were the same in the 2 sub-categories. Pseudo-IBC (11.1% of cases) can easily be confused with common true IBC. The biological characteristics of pseudo-IBC differ from those of true IBC: no dermal lymphatic involvement and little or no lymph-node involvement. Despite large tumour size, outcome was particularly favourable. It is of great importance to differentiate true and pseudo-IBC, for which the treatments are different. Confirmation of true IBC requires pathological demonstration of dermal lymphatic emboli or extensive lymph-node involvement. Occult IBC must be identified for patients presenting rapidly growing tumours. © 1995 Wiley-Liss, Inc.