To evaluate the outcome for osteosarcoma patients treated with different forms of adjuvant therapy in nonrandomized series, it is important to realize the possible influence of different prognostic factors. In two series of patients with primary classic osteosarcoma, survival was analyzed in relation to tumor size and site. In the first series comprising 57 patients, no adjuvant therapy was given and the five-year survival rate for all patients was 19%. A significantly (p less than 0.01) higher survival rate (43%) was noted for patients with tumors distally located and less than 0 cm in size (as measured by their largest diameter on radiograms), as compared to 12% for patients with larger and proximally located tumors. These figures were the same at three-and five-year follow-up. In the second series, comprising 22 patients, all received adjuvant interferon treatment. The three-year survival rate was 68%, and the five-year survival rate for the 12 patients followed was 58%. The survival rate for patients with tumors distally located and less than 10 cm in size was 88%, as compared to 57% for patients with larger and proximally located tumors. In the interferon series, tumor size appeared more important than location.