One hundred and sixty-one patients who were treated with nitrogen mustard, vincristine, procarbazine, and prednisolone (MOPP) chemotherapy for Hodgkin's disease have been observed for a median of 10.2 years. Eighty-two percent of those patients received MOPP after relapse from previous irradiation. The complete response (CR) rate was 71%. For the 116 patients achieving CR the relapse-free survival at 5 years was 83% and at 10 years, 79%. The overall survival was 72% at 5 years and 64% at 10 years. In a stepwise logistic regression analysis the most important clinical factors influencing response were B symptoms at presentation (fever greater than 38 degrees C, night sweats, weight loss greater than 10% of body weight), histologic subtype, and lung or pleural involvement. Patients who received MOPP as first-line therapy had a significantly worse response rate than those who received MOPP upon relapse after radiotherapy. This difference is reduced when adjustments are made for the presence of the above prognostic factors. A Cox regression analysis showed that Stage IV at presentation and lymphocyte-depleted histology were the most important factors indicating reduced survival. Patients who achieved a CR to MOPP had a significantly improved survival. Of the 65 patients who had died at the time of the analysis of this series, 46 died of progressive Hodgkin's disease. All four patients who developed secondary acute nonlymphocytic leukemia had received radiation as well as MOPP.