We studied 13 cases of malignant lymphoma involving the nasal cavity, in six men and seven women, from 27 to 92 years of age (mean, 56 years; median, 55 years). All lymphomas had a diffuse pattern, with 10 of large-cell type (six immunoblastic polymorphous, one immunoblastic, three large cleaved cell), one of mixed small- and large-cell type and one of small cleaved-cell type. One case could not be subclassified. Angioinvasion and prominent necrosis were seen in 10 cases. Pseudoepitheliomatous hyperplasia of the overlying epithelium was present in five cases. Immunohistochemical studies on frozen or paraffin sections in nine cases revealed that the atypical cells were T cells in four cases (CD8+ in two cases) and B cells with monotypic immunoglobulin in two cases. In three cases, the findings were suggestive but not diagnostic of T lineage. Genotypic analysis in one of two cases of T-cell lymphoma revealed clonal rearrangement of the genes for beta and gamma chains of the T-cell receptor. Patients were treated initially with local radiation therapy (10 cases) or with radiation and chemotherapy (three cases). Eight patients (62%) had no relapse and were free of disease between 9 months and 23 years (mean, 6 years 5 months; median 2 years 1 month) after diagnosis. Five patients developed recurrent disease, three of whom were successfully salvaged. One patient was alive with tumor at the time of last follow-up and one died with tumor. Among cases of malignant lymphoma presenting with involvement of the nasal cavity, we find a high proportion of angioinvasive, diffuse large-cell lymphomas, with a predominance of T-cell type, and a relatively good prognosis when treated with radiation therapy.