Eighteen patients with mediastinal involvement of Hodgkin disease were examined with magnetic resonance (MR) imaging before and during therapy to find out if size of residual masses could be predicted from the MR characteristics of the tumor at diagnosis. After the first treatment, a significant decrease in T2 values and signal intensity ratios of tumor to fat and tumor to muscle was found in all patients. There was no significant change in T1 values. The relative decrease in tumor size correlated well with signal intensity ratios and poorly with T2 values of the original tumor. No correlation with T1 values was found. The authors conclude that size of the residual mass can be predicted from the initial size of the tumor and the signal intensity ratios at diagnosis. Since the degree of low signal intensity in the tumor before treatment probably reflects the amount of fibrotic tissue, these results support the hypothesis that residual masses after treatment are remnants of the fibrotic stroma of the original tumor.