OBJECTIVE. The objective of our study was to determine the distribution of effective dose associated with a single low-dose CT chest examination of average-size participants in the National Lung Screening Trial. Organ doses were also investigated. MATERIALS AND METHODS. Thirty-three sites nationwide provided volume CT dose index (CTDI vol) data annually for the 97 MDCT scanners used to image 26,724 participants during the trial. The dose data were representative of the imaging protocols used by the sites for average-size participants. Effective doses were estimated first using the product of the dose-length product (CTDI vol x 35-cm scan length) and a published conversion factor, "k." The commercial software product CT-Expo was then used to estimate organ doses to males and females from the average CTDI vol. Applying tissue-weighting factors from both publication 60 and the more recent publication 103 of the International Commission on Radiological Protection (ICRP) allowed comparisons of effective doses to males and to females. RESULTS. The product of DLP and the k factor resulted in a mean effective dose of 1.4 mSv (SD = 0.5 mSv) for a low-dose chest examination across all scanners. The CT-Expo results based on ICRP 60 tissue-weighting factors yielded effective doses of 1.6 and 2.1 mSv for males and females, respectively, whereas CT-Expo results based on ICRP 103 tissue-weighting factors resulted in effective doses of 1.6 and 2.4 mSv, respectively. CONCLUSION. Acceptable chest CT screening can be accomplished at an overall average effective dose of approximately 2 mSv as compared with an average effective dose of 7 mSv for a typical standard-dose chest CT examination. © American Roentgen Ray Society.