During the period January 1983 to January 1988 1597 newly diagnosed cases of non-Hodgkin's lymphoma (NHL) were included in a Western Danish population-based NHL registry. Of these, 31% (N = 496) were low-grade NHL (LG-NHL) consisting of (Kiel): 9% lymphocytic (LY), 27% lymphoplasmacytic/-cytoid (IC), 53% follicular centroblastic/-centrocytic (CB/CCf) and 11% unclassifiable low-grade. LG-NHL (age range: 26-94 yrs, median: 64 yrs; M/F ratio: 0.8) had an age-standardised incidence rate (IR) of 2.7/10(5)/yr. Age-specific IR's showed an age-related exponential rise in all subtypes except for CB/CCf. Compared with the intermediate (IG)- and high-grade (HG) group, LG-NHL had more female cases (M/F ratio: 0.79 vs. 1.2; p = 0.0002), a higher frequency of stage III-IV disease (66% vs. 53%; p < 0.00005) and of bone marrow involvement (39% vs. 19%; p < 0.00005). A later revision of all IC cases (N = 132) distinguished 79 non-polymorphic (ICnp) from 25 polymorphic (ICp) cases; 28 cases were differently classified. In 34 LG-NHL patients histologic transformation was verified: CB/CCf to CB diffuse (22 pts) and LY to immunoblastic or CB type (6 pts). The 7-yr survival for LG-NHL was 63% (IG: 48%, HG: 38%; p < 0.00005). A Cox-regression analysis identified the following adverse prognostic factors for survival in LG-NHL: age > 50 with a relative risk (RR) of 3.2, hepatic involvement (RR = 2.1), elevated s-LDH (RR = 1.9), B-symptoms (RR = 1.8) and IC histology (ICnp+ICp) (RR = 1.7).(ABSTRACT TRUNCATED AT 250 WORDS)