We tried to correct the partial volume effect for pulmonary mass lesions using a 68Ga/68Ge transmission scan. Initially, a fundamental study was done using spherical phantoms and good results were obtained. Next, a clinical evaluation was performed on 28 pulmonary mass lesions ranging in diameter from 0.8 cm to 8.0 cm. The tissue fractions of the mass lesions were obtained by a 68Ga/68Ge transmission scan, where the tissue fraction in the back muscles was assumed to have a value of 1. The tissue fraction of the surrounding lung field was subtracted from that of each mass lesions, and the net tissue fraction was thereby obtained. When no correction was required, the tumor/muscle count ratio (TMR) became higher in proportion with an increase in the mass size. After performing a correction, however, no correlation was observed between them. This method was both easy to perform and reliable, and it is thus considered to be useful for overcoming the partial volume effect in pulmonary mass lesions, although an underestimation may still occur in cases with mass lesions either near the great vessels or the chest wall.