There are a variety of independent methods of estimating smoking status; measurement of expired air carbon monoxide has proven to be reliable and convenient. This article describes a new, inexpensive, portable analyzer for measuring expired-air carbon monoxide --the Bedfont EC50. We have compared this analyzer with the standard instrument for measuring expired-air carbon monoxide in 138 normal subjects and found good correlation and agreement. The Bedfont was demonstrated to be better at identifying smoking status, and the relevance of this finding for classification of smokers and nonsmokers by fixed-threshold carbon monoxide levels is discussed. The features of the Bedfont EC50 are particularly appealing for field and clinical use.