The objective of our work was to know the relationship between carbon monoxide (CO) levels in expired air and smoking habits among school youths and the relationships that can be established between CO level and some peculiar attitudes regarding consume by youths, such as the number of cigarettes, inhaling technique and time elapsed since the last cigarette was smoked. The study, of cross-sectional design, was performed in two high school centres and a total of 777 students who answered a questionnaire and had an expired air CO sample in their own schoolroom tested were enrolled. CO determination in the schoolroom was a simple and attainable technique for the pupils, as only 32 cases (4.1%) had to be excluded due to poor collaboration or poor technique. The mean (mean and SD) CO level in the control group (n = 247), made up by non-smokers nor tobacco tasters was 4.75 (2.46) ppm, statistically lower than among smokers (p < 0.001), but with no differences compared with non smokers (n = 563), who had a CO level of 5.23 (3.4) ppm. This figure was also lower (p < 0.001) than that obtained in the smokers (12.6 [6.3] ppm), made up of 214 pupils, with a mean consume of 2.7 (1.69) cigarettes/day. Among smokers the mean abstinence time since the last cigarette was smoked was 26 (44) minutes and 54% of them admitted to have smoked in the last 10 minutes. CO in expired air correlated significantly with the number of smoked cigarettes (r = 0.58; p < 0.001). Likewise, it correlated significantly with abstinence minutes (r = -0.38; p < 0.001). The time required for CO level to decrease below 10 ppm was 140 minutes in four cases and 120 minutes in 33 cases.