Twenty cases of cutaneous follicular lymphoid hyperplasia with monotypic plasma cells are presented in a clinicopathologic study on 18 patients. The plaque-like or nodular lesions were solitary in 10 and multiple in eight patients. Immunohistochemistry showed well-defined B- and T-cell areas. Sheets of monotypic plasma cells occurred either interfollicularly or adjacent to the sclerotic stroma, with expression of IgG/kappa in 14 and IgG/lambda in six cases. In one patient with multiple lesions, one sample contained polyclonal plasma cells, whereas the other specimen showed light chain restriction. In another patient, disease recurred with a polytypic cutaneous plasma cell infiltrate. Polymerase chain reaction (PCR) revealed clonal immunoglobulin heavy chain gene rearrangements in eight of 13 cases, which was confirmed by Southern blot analysis in three samples. Clonal T-cell receptor chain gene rearrangements were not detected. Disease progression to overt malignant lymphoma did not occur within the follow-up period of up to 12 years, but recurrent disease was seen in three patients. Our data indicate that cutaneous lymphoid hyperplasia with monotypic plasma cells is a biologically distinct clinicopathological entity.