From 1964 to 1987, the EORTC Lymphoma Group conducted four consecutive controlled clinical trials on clinical stages I and II Hodgkin's disease in which 1,579 patients were entered. From the onset the main aim of these trials was to identify the subsets of patients who could be treated safely by regional radiotherapy (RT). Therefore, several prognostic indicators were prospectively registered and progressively used in the trial protocols for the delineation of the favorable and unfavorable subgroups as soon as they were recognized of high predictive value. In the H2 trial (1972 to 1976), the histologic subtype was the only variable taken into account for the therapeutic strategy and the staging laparotomy findings were found to be of prognostic value only in patients with favorable prognostic indicators. In the H5 trial (1977 to 1982), patients were subdivided into two subgroups according to six prognostic indicators. Patients with favorable features were submitted to a staging laparotomy (lap); lap negative patients were randomized between mantle field RT and mantle field plus paraaortic RT. Disease free survival (DFS) and total survival (S) were similar in the two arms. Among patients with unfavorable features, DFS and S were significantly higher in the arm treated by combination of mechlorethamine, vincristine, procarbazine, prednisone (MOPP) chemotherapy (CT) and RT than in the arm treated by total nodal irradiation. Nevertheless, in patients below the age of 40, the overall survival rates were equivalent in the two arms. In the H6 trial, the delineation of the favorable subgroup was based on (a) absence of systemic symptoms and elevated ESR, (b) no more than one or two lymph node areas involved. The aim of the study was to assess the impact on survival of a therapeutic strategy including staging laparotomy. At a 4-year follow-up, no difference in survival was evidenced. In patients with unfavorable prognostic indicators, 3 MOPP-RT-3 MOPP were compared with 3 ABVD-RT-3 ABVD. From H1 to H5 trials, the proportion of patients having received CT during the course of the disease gradually decreased; the data suggest that a further reduction in the proportion of patients aggressively treated is conceptually possible. On the basis of the prognostic factors identified, one can delineate three subsets of patients and modulate toxic cost of the initial treatment according to the characteristics of these subsets. In the most favorable subgroup, RT alone produces high survival and CT is not justified.(ABSTRACT TRUNCATED AT 400 WORDS)