The OLINDA/EXM version 1.0 personal computer code was created as a replacement for the widely used MIRDOSE3.1 code. This paper documents the basic function of the code and how it is similar to and different from the MIRDOSE software. After creation of the code and alpha- and beta-testing phases, a premarket notification submission (510(k)) was filed with the Food and Drug Administration to permit marketing of the code. Permission was granted in June 2004, and the code is currently being distributed through Vanderbilt University. Not all of the technical details of the dosimetry methods have been shown here, as they have been previously documented. Agreement of doses between the MIRDOSE3.1 and OLINDA/EXM codes was good, within 1%-2% in most cases. The extensive testing of the OLINDA/EXM code, based on comparison with literature-established dose calculations and with the widely tested and accepted MIRDOSE3.1 code, should give users confidence in its output. The OLINDA/EXM code should be easy for MIRDOSE users to adopt and for new users to understand. It will be useful in standardizing and automating internal dose calculations, assessing doses in clinical trials with radiopharmaceuticals, making theoretic calculations for existing pharmaceuticals, teaching, and other purposes.