Structured questionnaires were used to gather data on the five target substances (alcohol, amphetamines, cannabis, cocaine, and ecstasy), recording patterns of substance use, adverse consequences from use, negative effects, functions for substance use, and perceived peer use. The study design was a cross-sectional survey. The study population was 16 to 22-year-old poly drug users. The findings indicate that functions for substance use strongly predicted intensity of use in all five substances when peer use, age of first use, and demographics were controlled. The analysis did support a relationship between social functions and intensity of use in any of the drugs. Functions concerned with relief from negative mood states were strong predictors of problem scores in alcohol, cannabis, and cocaine. It was concluded that the potential implications of using a functional approach to explaining and responding to substance use are considerable. This discovery aids in the understanding of how experimental substance use becomes regular and how regular use becomes problematic. This may have practical implications for health professionals that work with young people during the early stages of their drug-using careers. Instruments designed to profile patterns of substance use in young people could be enhanced by the inclusion of function measures in addition to more measures of frequency, quantity, and problems associated with use. 4 tables, 60 references