OBJECTIVE: To determine whether source of alcohol supply is related to adolescent underage drinkers' reports of risky drinking and alcohol-related problem behaviours. METHODS: In 2003/04, a cross-sectional survey of 2,644 16-17 year-olds were recruited from Victorian households and surveyed by phone as part of the Victorian Youth Alcohol and Drug Survey. The results were analysed to determine whether alcohol supply source was associated with weekly or more frequent risky single occasion drinking (RSOD) and reports of alcohol-related problem behaviours. RESULTS: Around 20% (524/2,644) of the sample reported weekly RSOD and 34% (904/2,644) of the sample reported engaging in at least one alcohol-related problem behaviour. These outcomes were associated with reported usual source of alcohol supply, with reports of alcohol sources in addition to parents alone more than twice as likely to be accompanied by reports of RSOD (OR=2.53, 95%CI=1.85-3.46) and/or problem behaviours (OR=2.16, 95%CI=1.64-2.84), compared to when adolescents reported parents as their sole source of alcohol. Reports of alcohol supply only from sources other than parents were similarly more than twice as likely to be accompanied by reports of RSOD (OR=2.27, 95% CI=1.74-2.95) and/or problem behaviours (OR=2.27, 95%CI=1.82-2.82) compared to compared to parental supply alone. CONCLUSIONS AND IMPLICATIONS: The rate at which older adolescents report RSOD and alcohol-related problem behaviour is increased when they obtain alcohol from sources other than their parents. Parents need to be equipped with strategies for managing the supply of alcohol to adolescents.