OBJECTIVE: To investigate the use of and expenditure on 17 of the most popular forms of complementary and alternative medicine (CAM) by adult Australians, sociodemographic characteristics of CAM users, and communication between CAM users and their doctors. METHODS: In May-June 2005, a sample of 1067 adults, 18 years and older, from all Australian states and territories, was recruited by random-digit telephone dialing and interviewed about their CAM use in the previous 12 months. RESULTS: In the 12-month period, 68.9% (95% CI: 66.1%-71.7%) of those interviewed used at least one of the 17 forms of CAM and 44.1% (95% confidence interval: 41.1%-47.1%) visited a CAM practitioner. The estimated number of visits to CAM practitioners by adult Australians in the 12-month period (69.2 million) was almost identical to the estimated number of visits to medical practitioners (69.3 million). The annual "out of pocket" expenditure on CAM, nationally, was estimated as 4.13 billion Australian dollars (US $3.12 billion). Less than half of the users always informed their medical practitioners about their use of CAM. The most common characteristics of CAM users were: age, 18-34; female; employed; well-educated; private health insurance coverage; and higher-than-average incomes. CONCLUSIONS: CAM use nationally in Australia appears to be considerably higher than estimated from previous Australian studies. This may reflect an increasing popularity of CAM; however, regional variations in CAM use and the broader range of CAM included in the current study may contribute to the difference. Most frequently, doctors would not appear to be aware of their patient use of CAM.