Using data from the National Survey of Mental Health and Well-Being (NSMHWB) collected in 1997 by the Australian Bureau of Statistics, this study examined: (1) patterns of co-morbidity between the levels of alcohol use; tobacco, cannabis and other substance use and disorders; and DSM IV cannabis; (2) patterns explained by common factors: demographic or neuroticism; and (3) if the presence of a co-morbid substance use disorder affected the likelihood of treatment seeking among people with alcohol use disorders. The study sample consisted of 10,641 residents in private dwellings aged 18 years and over across all States and Territories in Australia. Alcohol use was related strongly to the use of other substances. Those who did not report alcohol use within the past 12 months were less likely to report using tobacco, cannabis, sedatives, stimulants, or opiates. Co-morbid substance use disorders predicted a high likelihood of seeking treatment for a mental health problem among alcohol-dependent people. 2 Tables, 39 references