The objective of this study was to establish the association between substance abuse and the sociodemographic characteristics of secondary school students. All the students of 17 randomly stratified public secondary schools in Nairobi were required to complete self-administered sociodemographic and the School Toolkit questionnaires in a cross-sectional descriptive survey. Nearly all (96.6%; 1252/1296) the students, comprising more males (62.5%) than females, completed all the items on the questionnaires. Their mean age was 17 years. Alcohol and cigarette use were common and began as early as before age 11. No significant correlation was found between fathers' education and substance abuse. Mothers' education had a significant but negative correlation. There were increased rates of substance abuse compared to past surveys. Campaigns against drug use should include those aged 11 years and should focus on education on the dangers of alcohol and tobacco use, as these are gateways to use of other drugs. (Contains 6 tables.)