The analyses are based on a follow-up in 1980-81 of two cohorts representative of adolescents formerly enrolled in grades 10 and 11 in public secondary schools in New York State. Of the 1,626 former adolescents targeted for reinterviewing, 81 percent (1,325) were interviewed 9 years later at ages 24-25. Various models of progression were tested for their goodness of fit. The patterns formerly observed in adolescence that involved progression from one class of legal drug (either alcohol or cigarettes) to marijuana, to the use of other illicit drugs appear in the transitional period into young adulthood, with the additional stage of prescribed psychoactive drugs. There was a sex difference in the role of tobacco cigarette use in the progression of drug involvement among women than among men. The evidence for the existence of patterns of progression is stronger than could be derived from prior analyses of Guttman scaling of cross-sectional data or of short-term longitudinal data. The existence of sequential stages of drug-use progression, however, does not necessarily imply causal linkages among different drugs, since the observed sequences could reflect the association of each class of drugs with different ages of initiation and/or individual attributes rather than the effect of the use of one class of drug on the use of another drug. For the next article in this series, see NCJ-154310. 2 tables and 26 references