Primary leptomeningeal lymphoma: report of 9 cases, diagnosis with immunocytochemical analysis, and review of the literature. We describe 9 patients who presented with a neoplastic meningitis of lymphomatous origin. No evidence of parenchymal central nervous system or systemic tumor was identified either at the time of presentation or throughout the course of their disease. We have chosen to call this entity "primary leptomeningeal lymphoma" (PLML). This unusual form of neurologic lymphoma must be differentiated from the more common clinical situations of primary parenchymal lymphoma with meningeal involvement and systemic lymphoma complicated by lymphomatous meningitis. 