Twelve biopsied ocular adnexal benign lymphoid hyperplasias (OABLH) satisfying benign histological criteria were investigated for clonal immunoglobulin (Ig) heavy-chain gene rearrangement by means of a two-step polymerase chain reaction (PCR) method using formalin-fixed and paraffin-embedded tissue. Of the 12, 4 (33%) demonstrated clear single bands of the rearranged gene for the Ig heavy-chain, of between 100 and 150 base pairs. The selected cases were all free of malignant lymphoma and all of the lesions were small (2 x 2-22 x 6 mm; median 4.5 x 3 mm). Histopathological and cytological features were not essentially different between monoclonal and non-monoclonal examples. Immunohistochemistry was of little benefit in separating the two. It is concluded that OABLH demonstrating a benign clinical course frequently contain monoclonal B cell populations suggesting a continuous progressive spectrum of lesions in B cell neoplasia. In addition, the significance of molecular-genetic analysis for OABLH and the utility of the two-step PCR method should be emphasized.