Lymphoid tissues from 12 patients were diagnosed as reactive lymphoid hyperplasia, but surface immunoglobulin studies revealed monoclonal (single class) immunoglobulin staining patterns. Infectious, autoimmune, and immunodeficient conditions were diagnosed on the basis of histology and clinical features. Such surface immunoglobulin restriction has been used as an indicator of a neoplastic lymphoid proliferation, but the cases of these patients, in whom the histologic diagnosis was benign, emphasize the importance of a multiparameter approach to diagnosis. Although at the time of this report none of the patients still available to follow-up study have developed known lymphoid neoplasms, the possibility that monoclonal SIg patterns are a harbinger of neoplastic disease makes continuing follow-up of such patients important.