Primary malignant deciduoid mesothelioma is a rare subtype of epithelioid mesothelioma that was first described in the peritoneum in young women without a history of asbestos exposure. It was thought to be a distinct clinicopathologic entity with ominous prognosis; recent studies have better characterized this entity. On morphology, primary malignant deciduoid mesothelioma is characterized by cytomorphologic features resembling decidualized tissue. Pleomorphism is variable. The immunoprofile is similar to other epithelioid mesotheliomas. The prognosis is the same as other epithelioid mesotheliomas and seems to depend on histological grade.