Despite the growing problem of global cigarette use, accurate information on the prevalence and pattern in Nigerian adolescents’ remains sparse. The aim of this study was to determine the prevalence of cigarette smoking among in-school male adolescents in Nigerian population and to assess their level of knowledge on the health effects of cigarette smoking. A descriptive cross-sectional study was carried out among 944 randomly selected in-school male adolescents in Owerri, south-east Nigeria, between September and November 2013. Information was obtained using a self-administered questionnaire. The findings indicated a prevalence of 15.3% for ever smoked adolescents and 11.2% for current smokers. The mean age at cigarette smoking initiation was 14 years. Sixty-three percent had good knowledge of health problems associated with cigarette smoking. Lung cancer was the most reported associated health problem. Fifty-two (36.1%) indicated that they were initiated by their friends who offered them their first stick of cigarette. The major reason for smoking cigarette the first time was to be like their friends/role model and out of curiosity. Poor knowledge of health effects of smoking was statistically associated with cigarette smoking (X2 = 26.82, p-value < 0.001). Knowledge of health problems associated with smoking proved to be the major reason for not smoking by never smoked adolescents, which means that awareness creation on health problems associated with cigarette smoking through health education in schools coupled with stiff legislative ban on sale of cigarette to adolescents will, to a reasonable extent, reduce the high prevalence of adolescent cigarette smoking in our society.