Studies of adolescent substance use progression typically infer a sequence of initiation from self-reported ages at first use of alcohol, tobacco, marijuana and hard drugs. The paper examines the reliability of this procedure for a sample of 892 New Jersey youths interviewed on two occasions separated by 3 years. Individual responses on the second occasion differed substantially from those provided on the first. However, the inferred sequences were consistent as long as first use of alcohol and/or tobacco was considered a single stage and cases in which individuals initiated the use of two substances in the same year were considered as ambiguous regarding order. The sequences reported were also consistent with the gateway theory that suggests alcohol/tobacco precedes any possible use of marijuana and hard drugs. Tables, figure, references