Five cases of angiomyolipoma (AML) composed exclusively or predominantly of smooth muscle cells (SMC) are presented to emphasize the histological diversity and to caution against errors in histological diagnosis. Four tumours were located in the liver and one in the renal capsule. Three patients were female and two were male, ranging from 36 to 76 years of age with a mean age of 50 years. One patient with a renal capsular tumour was associated with tuberous sclerosis. Two tumours were composed predominantly of a spindle-shaped SMC component, whereas three others were composed predominantly of epithelioid SMC elements. AMLs composed exclusively or predominantly of spindle-shaped smooth muscle cells (SMCs) stimulated leiomyoma, whereas AMLs composed exclusively or predominantly of epithelioid SMCs resembled epithelioid leiomyoma or leiomyosarcoma or other sarcoma when cellular atypia was present. However, both spindle and epithelioid SMCs were characteristically positive for HMB-45 melanoma-specific antibody; no other tissue components in either the liver or kidney were reactive to HMB-45. AML is often composed predominantly of SMC elements, and morphological features of the SMC elements are quite variable. Therefore, careful attention must be given to histological assessment of AML. Whenever a pathologist encounters an unfamiliar hepatic or renal tumour, the possibility of AML should be considered. Reactivity for HMB-45, however, confirmed the diagnosis of AML.