Immunologic analysis of cell surface markers (immunophenotyping) has become a standard procedure in the evaluation of systemic lymphomas. However, attempts to apply these techniques to intraocular lymphoma have not been uniformly successful. We successfully immunophenotyped five consecutive cases of intraocular lymphoma using immunoperoxidase surface marker analysis of cytocentrifuged specimens in two cases and flow cytometry in three. In all five cases, a monoclonal B-cell population was unequivocally present. Contrary to previous reports, we found surface marker analysis of vitreous biopsy specimens to be helpful in the diagnosis and treatment of intraocular lymphoma. Not only did it support the cytologic diagnosis but it allowed comparison of the immunophenotype of vitreous infiltrates with that of previous or subsequent lymphomatous lesions from nonocular sites.