A series of 60 patients with "high risk" Stage II and III Hodgkin's disease (B symptoms, or large mediastinal mass, or E lung disease) were staged without laparotomy and treated with combined modality treatment: mechlorethamine, vincristine, procarbazine, and prednisone (6 MOPP) plus radiotherapy. Patients were restaged after the first three courses of MOPP and the status of response to therapy at that time was called early response to chemotherapy (ERC). The rate of nitrogen mustard and procarbazine delivery (MRD) during the first three cycles of chemotherapy also was assessed. At the completion of the therapy patients were restaged and the final response was assessed. Fifty-two (86.7%) patients entered complete remission (CR). Forty-eight percent of the complete responders achieved CR in the first three courses of MOPP. Eight-year survival and disease-free survival (DFS) rates of the patients achieving CR were 71% and 73%, respectively. Survival and DFS were significantly better for the patients who achieved CR in the first three cycles of chemotherapy than for patients who entered CR at a later stage of therapy: 8-year survival 90% versus 55% (P = 0.00); 8-year DFS 87% versus 59% (P = 0.01). The attainment of a complete ERC was adversely affected by lymphocyte depletion (LD) histologic type (P = 0.01) and MRD less than 65% (P = 0.04). However, when a multivariate regression analysis was used, ERC was the only significant prognostic variable for survival and DFS and its predictive value was confirmed even after correction by MRD. These data suggest that the rapidity of response to chemotherapy could be an important prognostic factor in high-risk Stage II and III Hodgkin's disease.