To learn more about the natural history of low-grade non-Hodgkin's lymphoma, we have studied 83 patients in whom the advanced disease was initially managed without therapy. Actuarial survival was 82 per cent at 5 years and 73 per cent at 10 years. The median time until therapy was required was three years. Spontaneous regressions occurred in 19 untreated patients (23 per cent), including 30 per cent of patients with nodular, poorly differentiated lymphocytic lymphoma. Histologic transformation to an intermediate-grade or high-grade lymphoma occurred both before and after primary therapy. The actuarial risk of transformation among the initially untreated patients was similar to that in a group of patients treated at this institution immediately after diagnosis. Neither the time to histologic transformation nor the incidence of transformation was influenced by when therapy was started.