Ten examples of a pseudosarcomatous myofibroblastic proliferation occurring in the urinary bladder of children (aged 2 to 16 years) are reported. The lesions appeared as polypoid nodular masses of variable size with myxoid and hemorrhagic areas. They consisted of compact fascicles of elongated spindle cells with minimal atypia. Myxoid areas of variable extension and scattered inflammatory cells were constant features, whereas diffuse collagen deposition was not common. Despite the striking cellularity of some of the lesions, most showed minimal mitotic activity. Ultrastructurally, the predominant cells had features of myofibroblasts. Six cases studied by immunocytochemical methods expressed vimentin and muscle-specific actin. In addition, two of these cases expressed desmin and two others cytokeratin. Infiltration into the muscularis propria of the urinary bladder was demonstrated in six cases and into the perivesical soft tissues in two. However, none of the eight patients for whom follow-up information is available has had local recurrence or metastasis develop 18 months to 6 years after surgical excision.