Mutagenic and/or carcinogenic substances, which are present in the environment, are numerous. They include electrophilic compounds, polycyclic aromatic compounds such as benzo(a)pyrene which is a polyaromatic hydrocarbon. It is well known to be mutagenic and carcinogenic. Biological monitoring that depends on estimating a substance or one of its metabolites or one of its effective components in biological fluids such as urine and blood was carried out. Thioether concentration in urine indicates absorption of mutagenic and/or carcinogenic substances. Thiocyanates' concentration in urine was used as an indicator of absorption of mutagenic and/or carcinogenic haloacetonitriles. Urinary cotinine was used to estimate the extent of exposure to tobacco smoke, the heaviness of smoking, and whether the subject is smoker or not. Three hundreds and thirty two male students were randomly selected from eight secondary schools in Jeddah city. Their ages were more than 15 years and they were apparently free from illness. Every student filled a questionnaire. Random urine samples were obtained from all students. Thioether, thiocyanate, cotinine, and creatinine were measured in urine samples. There was a direct relation between urinary thioether and each of smoking index, food index, urinary thiocyanate, and cotinine. Urinary thioether shows an inverse relation with BMI. Urinary thiocyanate and cotinine showed direct relation with smoking index and with each of them and the other factor. This indicates interference between these indices and smoking is still one of the most misleading factors in researches that deal with the mutagenic and/or carcinogenic effects in the environment.