The natural history of lymphoid infiltrates of the orbit and conjunctiva is poorly understood. To determine if immunohistologic features could predict clinical outcome, these features were evaluated in 61 patients with orbital and conjunctival lymphoid infiltrates, including 44 patients with lesions localized to one or both orbits or conjunctivae. Using histologic criteria, 20 infiltrates were classified as malignant lymphoma (cytologically atypical), 14 cases were classified as benign (follicular hyperplasia or inflammatory pseudotumor), and 27 infiltrates (44%) were dense infiltrates of small lymphocytes without cytologic atypia, which could not be confidently classified as benign or malignant (histologically indeterminate). Based on expression of monotypic immunoglobulin, 20 of these cases were reclassified as small lymphocytic lymphoma. For all cases, monotypic immunoglobulin expression correlated with reduced survival (P less than .05) and increased likelihood of dissemination (P less than .001). Monotypic immunoglobulin expression also correlated with increased risk of dissemination for all histologically indeterminate (small lymphocytic) infiltrates (P less than .05). Separate analysis of the localized infiltrates showed that monotypic immunoglobulin expression significantly correlated with an increased likelihood of dissemination for all 44 cases (P less than .005) and for the 22 histologically indeterminate lesions (P = .06). For the localized small lymphocytic infiltrates, monotypic immunoglobulin expression conferred a 50% risk of dissemination. In contrast, no patients with polytypic small lymphocytic infiltrates have disseminated, although one lesion locally recurred 30 months later as lymphoma. Thus, monotypic immunoglobulin expression significantly correlates with reduced patient survival and increased risk of dissemination by orbital and conjunctival lymphoid infiltrates. Monotypic small lymphocytic infiltrates without cytologic atypia behave as do low grade B-cell malignant lymphomas with a significant risk of dissemination and an indolent clinical course. The relationship of polytypic small lymphocytic infiltrates to lymphoma remains to be determined.