Dose distribution about an 125I source in tissue was calculated using absorbed dose constant and point isotropic source buildup factors. The effect of encapsulation material was included. Depth-dose values were verified by measurements with thermoluminescent dosimeters. A value for absorbed dose constant of 1.32 rad cm2/mCi-h was found, compared to the presently used value of 1.7. Depth-dose values were also discussed.