In this analysis, we compare the patterns of failure in the first 12 months for 660 eligible patients randomized to two Southeastern Cancer Study Group (SECSG) protocols for limited extent, small cell carcinoma of the lung between 1978 and 1985. In each protocol, a different schedule of radiotherapy was given in conjunction with combination chemotherapy and was compared with combination chemotherapy alone. In protocol 78 LUN 328, radiotherapy was given between courses of chemotherapy, and in protocol LUN 81343, it was given simultaneously. The rates of local failure, either as an initial or subsequent site, in the first 12 months were significantly lower when thoracic irradiation was given than when it was not (P less than .01). When the 2 radiotherapy arms were compared, there were no significant differences in the rates of local failure alone, but a smaller proportion of patients developed both local failure and distant metastases (P less than .01) when simultaneous radiotherapy was administered. Survival on all 4 arms was similar during the first 2 years of patient study. After 2 years, both radiotherapy regimens showed a trend toward improved survival compared with the combination drug alone (cyclophosphamide, doxorubicin, vincristine) arms. On both protocols, survival from 12 months was significantly longer for those with local control at 12 months than for those who did not show local control.