The effective half-life (Teff) of radioactivity taken up by residual thyroid tumor after 131I therapy was measured and compared with response to treatment. Teff was determined from uptake by the lesions at four, eleven, and 18 days after 131I administration. The criterion for successful 131I therapy was that, in any successive 131I therapy, lesion uptake in the second therapy decreased to less than one-twentieth of that in the first therapy. In the 26 patients studied, 48 lesions were evaluated. Therapy was considered successful in 27 of these lesions and unsuccessful in 21. Mean Teff in the former was 2.48 +/- 0.93 days (mean +/- S.D.), significantly shorter than that in the latter, 5.70 +/- 2.62 days (p < 0.001). In some of the cases of solitary lesion, serum TSH levels, serum thyroglobulin values, and 131I protein bounded iodine (131I-PBI) were measured, but there were no significant differences between successful and unsuccessful therapy. The results of this study suggest that Teff seems to reflect the functional nature of the lesion and could be of prognostic value for the response to 131I therapy.