Bone marrow depression following 131I therapy for metastatic thyroid cancer can occur in up to one-quarter of all patients so treated. An analysis was made of the 131I whole body (WB) retention and its relationship to activity in blood for 46 patients (45 adult, 1 adolescent in 49 total studies) to define the accuracy of utilizing WB external counting data as a predictor of blood dose in comparison to the more classical method which requires data from sequential blood samples. The mean percentage differences between blood dose estimates based on external WB counting and those calculated by the classical method lie within +/- 10%. The WB methodology provides a useful first-order approximation for hematopoietic dose estimates in adult patients undergoing 131I therapy for thyroid cancer.