Sixty cases of orbital lymphoid neoplasms originally accessioned as malignant lymphomas were analyzed histopathologically and the follow-up data evaluated separately for each category as follows: inflammatory pseudo-tumor, 5; reactive lymphoid hyperplasia, 8; atypical lymphoid hyperplasia, 7; and malignant lymphocytic lymphoma (classified according to Rappaport), 40. Significant clinical differences were not observed among these patients, suggesting that the lesions must be distinguished by histologic rather than clinical criteria. Systematic application of the histologic criteria discussed here improved diagnostic accuracy and our ability to predict clinical outcome as substantiated by follow-up data. Two of 13 patients (15%) with benign pseudo-lymphomas, 2 of 7 patients (29%) with atypical lymphoid hyperplasia, 2 of 8 patients (25%) with well differentiated lymphocytic lymphomas, and 22 of 32 patients (68%) with less differentiated lymphocytic lymphomas either had or later developed systemic lymphomas. Thus, the degree of cytologic differentiation appears to be the single most important factor for determining the prognosis of patients with orbital lymphoid neoplasms.