Although peripheral primitive neuroectodermal tumour (pPNET) and extra-osseous Ewing's sarcoma (EES) are thought to be closely related neoplasms, their clinical behaviour differs considerably. To determine the clinical relevance of the Schmidt classification scheme for differentiating pPNET and EES, 20 tumour specimens of poorly differentiated round cell tumours were evaluated. In addition, the diagnostic value of several neural markers and the prognostic value of quantitative morphological variables (DNA ploidy, S-phase fraction, and the mitotic activity) were assessed. Homer-Wright rosettes were present in 9 tumours. Neuron specific enolase (NSE) was expressed in 11 tumours, 8 of which expressed a second neural marker (CD57, S100, or neurofilament). According to the Schmidt classification, 11 pPNET and 5 EES were distinguished. HBA-71 was exclusively expressed in pPNET and EES. The remaining tumours were classified as sarcoma not otherwise specified (n = 2), rhabdomyosarcoma (n = 1), and desmoplastic tumour with divergent differentiation (n = 1). EES611 patients fared significantly better than the pPNET patients (100% versus 42% 5-year survival). Neither DNA ploidy nor S-phase fraction assessed in 12 evaluative histograms (9 pPNET and 3 EES), nor mitotic activity yielded information of additional prognostic value. On the basis of this study and the Schmidt classification scheme, it can be concluded that if the diagnosis of EES and pPNET is based on light microscopy (Homer-Wright rosettes) and/or immunohistochemistry (at least two neural markers, i.e. NSE, S-100, CD57, and neurofilament), the classification provides important clinical information. Furthermore, positivity for HBA-71 is helpful in differentiating pPNET and EES from all other small round cell tumours.