Purpose In this study on a series of 205 patients with rhabdomyosarcoma, we investigated whether the prognostic effect of tumor size, at diagnosis or in terms of tumor response after induction chemotherapy, differed when tumor diameter or tumor volume were considered. Patients and Methods Tumor size was assessed radiologically at diagnosis and, for the 108 patients with measurable disease, after three courses of chemotherapy. The analysis was based on multivariable models (linear for association between size and patient/tumor characteristics, Cox for association with survival). The predictive performance of the Cox model (estimated by V measure) was compared for the tumor's diameter and volume. Results Initial tumor size was significantly larger in male or older patients and in T2 or alveolar tumors, but was not associated with the achievement of complete surgical resection. Initial tumor size significantly influenced overall survival. The risk of death was comparable for tumors 10 cm in maximum diameter and 194.0 cm³ in volume. The predictive performance of the Cox model was much the same when the tumor's diameter or volume was considered. Tumor response was a significant predictor of survival, again irrespective of the type of tumor measurement considered. Conclusion In our analysis, initial tumor size and tumor response were significant prognostic factors in rhabdomyosarcoma, regardless of whether tumor diameter or volume was considered. Three-dimensional tumor assessment was of no greater prognostic value than one-dimensional assessment, neither initially nor after induction treatment.