Background Alcoholism is one of the antisocial activity which exists all over the world. Worldwide, 3.3 million deaths every year result from harmful use of alcohol,2this represent 5.9 % of all deaths. In the age group 20 – 39 years approximately 25 % of the total deaths are alcohol-attributable. Tobacco kills more than 7 million people each year. More than 6 million of those deaths are the result of direct tobacco use while around 890 000 are the result of non-smokers being exposed to second-hand smoke. This study aims to assess the knowledge of alcoholism and smoking among adolescent boys in Chennai.  Methods The study was carried out with self-structured questionnaire related to alcoholism and smoking.  Results The results of the studyshow that 66.7% have moderately adequate knowledge and 29.3% have adequate knowledge and 4% had inadequate knowledge regarding effects of alcoholism and smoking.