The labeling index (LI) was determined at the time of initial diagnosis of 88 untreated adults with non-Hodgkin's lymphomas. The frequency distribution of low (less than or equal to 4%) and high (> 4%) LI between tumors with nodular and diffuse histologic patterns was significantly different (P < 0.001): The median LI was about four times greater for diffuse lymphomas than for nodular lymphomas. LI was not correlated with either disease extent or systemic symptoms. In patients with tumors of low LI, complete remission occurred in 86% of those with modular lymphomas and in 74% of those with diffuse lymphomas. These values were significantly different from the 17 and 39% observed respectively for nodular and diffuse lymphomas with high LI. Patients with low LI had greater actuarial 4-year survival than did those with high LI (P < 10(-7)). Highly significant differences between the two kinetic groups were also observed within nodular and diffuse patterns. However, whereas the difference for nodular lymphomas was independent of histologic subtype and stage, this was not always true for all subtypes of diffuse lymphomas. LI could be a useful marker to identify prior to treatment the patients who could enter complete remission and have favorable survival. Its clinical relevance as a prognostic factor appeared superior to the Rappaport histologic classification and pathologic stage, especially in patients with nodular histology.