We correlated the light microscopic features of ten ocular adnexal lymphoid neoplasms with the results of selected immunologic and histochemical lymphocyte marker studies. The lesions were divided into three histomorphologic patterns: inflammatory pseudotumor, reactive follicular hyperplasia, and malignant lymphoma. The six inflammatory pseudotumors, and reactive follicular hyperplasia, despite diverse histomorphology, were immunologically polyclonal and consisted of from 45 to 73% T cells and 27 to 60% B cells, the B cells being polyclonal with respect to light chain determinants. The four malignant lymphomas were immunologically monoclonal. Each consisted of a predominant B-cell proliferation, greater than 50% of the cells, which were monoclonal with respect to surface light chain determinants. No T-cell proliferations were observed. Our preliminary data indicate that, as in systemic lymphoid neoplasms, benign reactive lymphoid hyperplasias are immunologically polyclonal whereas malignant lymphomas are immunologically monoclonal. The systematic application of immunologic and histochemical techniques to the study of ocular adnexal lymphoid neoplasms may elucidate the natural history of these lesions.