Preoperative CT volumetry was performed in 39 patients with intrathoracic neoplasms. The results are compared to postoperative volume measures of the surgical preparation. The correlation of both measuring series was very high (p = .94). The estimated influence of systematic and stochastic errors was only 10 to 15%. The authors outline the consequences for clinical practice: by means of the better quantification in the T category of the TNM system, the surgeon is in a position to make a more precise preoperative planning. The reliable numeric in vivo determination of regression and growth velocity of tumors and/or metastases allows for the first time an exact quantification of control examinations during and after radiotherapy or chemotherapy. This will possibly lead to a more precise individual prognosis. In experimental radiotherapy a more detailed description of the evolution of experimental tumors in the living animal will be possible by this procedure.