In a retrospective analysis on 128 patients from the trials COSS-80, -82, -85 and -86 initial x-ray pictures were evaluated for tumor diameters in three planes and the prognostic meaning on survival was assessed. In a subset of patients (n = 27) the measured values were compared to values obtained by CT-Scan and a good correlation (r = 0.69) was found. Several parameters for tumor size were defined: absolute tumor length (ATL), relative tumor length (RTL, proportion of tumor to the length of the involved bone), absolute tumor volume (ATV, calculated by the ellipsoid formula) and relative tumor volume (RTL, tumor volume referred to the body surface area) and univariate and multivariate survival analysis were performed. Univariate analysis of metastasis free survival (MFS) revealed a high prognostic significance of the ATL, the ATV and the RTV. The RTL in this patient group demonstrated a tendency only toward an inferior prognosis in larger tumors. None of the patients with a ATV < 70 ml (n = 19) and only one of 33 patients with an ATV < 100 ml relapsed. Cox regression analysis was performed including the variables age, sex, site and response (> 90% tumor necrosis) in 84 patients. ATL and RTL do not enter the model, while the response proves its significance as a valid prognostic factor with a p-value of 0.0004. Adding the ATV as the measure of tumor size to the model it enters as the first term (p = 0.0000) followed by the response (p = 0.0002).(ABSTRACT TRUNCATED AT 250 WORDS)