The classification of the histologic types of rhabdomyosarcoma is based on poorly defined criteria. This has resulted in marked disparities in studies reported from different institutions, as well as difficulties in assessment of the clinical behavior of the different histologic types. A retrospective morphologic analysis of 36 consecutive cases of rhabdomyosarcoma of childhood was undertaken according to predefined and strict guidelines for diagnosis. Undeflecting adherence to such criteria identified the embryonal type as the most common form, and the alveolar variant as a distinct clinicopathologic entity with a much more aggressive course; it also resulted in a large proportion (approximately one-fourth) of sarcomas of undetermined histogenesis. In spite of either prolonged follow-up observation with repeated biopsies, autopsy study, or electron-microscopic study of tumor tissue, no evidence could be obtained to substantiate the rhabdomyogenic derivation of the latter group of neoplasms. Precise systematization of the morphology of these cases may be contingent upon careful inventory of their fine structural features; current classifications appear to have disregarded the morphologic heterogeneity of this group of tumors.