We describe four patients with retroperitoneal liposarcomas undergoing myosarcomatous differentiation. The patients (two men and two women) were 47, 48, 68, and 72 years of age when first seen. The primary tumors were large retroperitoneal, well-differentiated liposarcomas, one featuring areas of dedifferentiation (without muscle elements). A myosarcomatous component became evident at the first recurrence in three cases and at the second recurrence in one. This component was always within dedifferentiated areas and in three of the cases coincided with the emergence of the latter. The muscle component had exclusively leiomyosarcomatous phenotype (alpha-smooth-muscle actin reactivity) in one case, exclusively rhabdomyosarcomatous phenotype (myoglobin reactivity) in two cases, and combined leiomyosarcomatous and rhabdomyosarcomatous phenotype (alpha-smooth-muscle actin and myoglobin) in one case. Ultrastructural studies of one of the tumors with a rhabdomyosarcomatous component revealed the presence of sarcomeres. Two patients died of extensive retroperitoneal disease, one patient died following the attempted removal of a recurrence, and one patient is alive and free of disease. These cases demonstrate that the dedifferentiated component of liposarcoma may exhibit a myosarcomatous component, a feature analogous to that previously described in dedifferentiated chondrosarcoma.