This study examined the relationship between self-reports of marijuana and cigarette smoking, and the physiological measures of expired air carbon monoxide (CO) and saliva thiocyanate (SCN) in a sample of 1,130 seventh, ninth, tenth, eleventh, and twelfth graders. Subjects who reported marijuana smoking were likely to also report cigarette smoking. The correlation between self-reported marijuana smoking and SCN was negligible. The correlation between CO and self-reported marijuana smoking was statistically significant, but when the variance due to cigarette smoking was removed, this relationship also became negligible. The existence of a sizeable number of marijuana smokers in this sample of adolescents did not alter the correlation between CO and self-reports of cigarette smoking. However, in adult samples, where marijuana and cigarette smoking are less highly correlated, marijuana smoking could affect the relationship between CO and self-reported cigarette smoking. Carbon monoxide predicted self-reported cigarette smoking better than did saliva thiocyanate. There was an interaction between grade and the CO/cigarette smoking correlation. The correlations were generally higher in upper grades.