Seventy-five malignant mesotheliomas of the peritoneum in women were reviewed to highlight their morphologic spectrum. The patients ranged from 17 to 92 (mean, 47.4) years of age. The clinical presentation was usually abdominal or pelvic pain, abdominal swelling (sometimes due to ascites), or a pelvic mass. On microscopic examination, the majority of the tumors had only an epithelial morphology, but 4 were biphasic and 1 was sarcomatoid. The most common epithelial patterns were tubular and papillary (which often coexisted), but 5 tumors were purely diffuse; 2 had cells with abundant glassy eosinophilic cytoplasm (so-called deciduoid mesothelioma). The cells in the tubular and papillary patterns were generally cuboidal with scant to moderate amounts of eosinophilic cytoplasm. Nuclear atypia was usually only mild, although a minority of cases had moderate or even, occasionally, severe atypia. Many tumors had foci that, viewed in isolation, resembled so-called well-differentiated papillary mesothelioma, and accordingly that diagnosis should be made cautiously. Unusual features were lymphoid follicles (13 cases), striking myxoid stroma (5 cases), prominent foamy histiocytes (5 cases), and a striking vascular proliferation (1 case). The varied morphology of peritoneal malignant mesotheliomas may raise a broad differential diagnosis, but in most cases the resemblance to other tumors is limited. Histochemistry, immunohistochemistry, and electron microscopy may provide important aid, particularly when tissue is limited, but should be needed only occasionally.