Percent carboxyhemoglobin (%COHb) serves as a marker for carbon monoxide (CO) uptake. Commonly cited reports suggest %COHb concentrations in smokers of 4-10%, with 10% suggestive of "heavy" smoking. However, %COHb measures in smokers are influenced by a number of factors including sample time of day and number of cigarettes smoked prior to testing. We summarize here %COHb data gathered from 43 separate studies conducted in our laboratory over several years using a standardized afternoon sampling protocol. Many of the studies also included expired air CO measures to compare to the %COHb determinations. Whole blood %COHb concentrations were determined between 12:00 and 17:00 in a total of 734 participants smoking their usual brand (UB) cigarette. Subjects were tested in a laboratory setting with samples drawn 25 min after lighting the cigarette. Mean %COHb concentrations were 8.0% in female smokers (628 observations) and 7.5% in male smokers (592 observations). In addition, approximately 25% of female smokers and 15% of male smokers had afternoon %COHb levels greater than 10%. One female and one male smoker exceeded 17% COHb concentration. Our mean %COHb data trend to the high side of the reported literature values for smokers, and would typically be interpreted as indicating a sample of "heavy" smokers. However, the smokers in these studies achieved these %COHb concentrations after smoking an average of only 12.3 cigarettes prior to the test session. We believe the higher %COHb concentrations seen in our work (compared to commonly cited clinical data) principally reflect a difference in the time of day the samples are taken, since clinical observations are most often made in the morning hours. In addition, our data suggest that the practice of using expired air CO measures to predict %COHb concentrations should be avoided if precise comparisons of changes in CO uptake are to be made.