We describe nine patients with a primary Ki-1 (CD30)+ T-cell lymphoma containing numerous, often CD30-negative, small lymphocytes with irregular nuclei and a minor population of large CD30+ tumor cells. All previously described primary Ki-1+ lymphomas have been large-cell neoplasms. In this small-cell variant, the diagnosis of lymphoma was difficult to make because there was a predominance of small lymphocytes and, in some cases, clinical features suggested an inflammatory process. Patients were young (age range 0.3-40 years, median 14 years), and frequently had B symptoms (56%); sites of involvement were predominantly skin (78%) and lymph node (67%). The actuarial 2-year disease-free survival was 14%, and the overall survival was 51%. Two patients had a rapidly fatal course. In all cases histologic sections showed a predominance of small lymphocytes with marked nuclear irregularity and often a perivascular/intravascular distribution of CD30+ large cells. All cases had a T-cell phenotype. In four cases the large and small cells could be compared and had a similar aberrant T-cell phenotype. Large cells were CD30+, but only rare small cells expressed CD30. Cytogenetic studies revealed a t(2;5)(p23;q35) in four of four cases studied. Four patients had numerous large cells on repeat biopsies; two of these developed sheets of large CD30+ cells typical of anaplastic large-cell lymphoma (ALCL). These cases provide further evidence that primary Ki-1+ lymphoma has a morphologic spectrum that includes a small-cell variant. Although very different morphologically from previously described Ki-1+ ALCL, this small-cell variant is clearly part of the disease spectrum on the basis of clinical features, the presence of the t(2;5)(p23;q35), the aberrant T-cell phenotype in the small and large cells, as well as histologic progression seen in several patients.