An analysis of 318 permanent implants was carried out in an attempt to establish the effectiveness of iodine 125 as compared with radon 222 and iridium 192; and to derive dose-response curves for iodine 125. Analysis of the clinical results obtained with iodine 125 in comparison with radon 222 and iridium 192 in selected anatomical sites demonstrated that iodine 125 has a higher therapeutic ratio than the other two radionuclides. A tumor control rate of at least 80% with iodine 125 implants might be expected after a good geometrical dose distribution, with a planned initial minimum tumor dose of at least 14,000 +/- 2,000 rads. Rapid tumor shrinkage ensures a more or less steady or even increased dose rate over a period of several weeks and improves the local tumor control. Tumors with a longer cell cycle are especially susceptible to irradiation with radionuclides of long half-lives.