Objective: To compare Medicare-related costs and service utilisation of people with diagnosed diabetes, asthma or chronic obstructive pulmonary disease (COPD) to those who were previously undiagnosed, and those without these conditions. Design, setting and participants: Representative cross-sectional study of people (18+ years) living in the north-west area of Adelaide. Participants were recruited by telephone interviews. Biomedical and self-report data for 2352 participants were linked to Medicare Australia Medicare Benefits Schedule (MBS) data from 1997 to 2002. Main outcome measures: Mean number and cost (benefit paid) of MBS services for people with diagnosed and previously undiagnosed diabetes, asthma, and COPD, and those without these conditions. Results: Mean (±SD) MBS costs were significantly greater for people diagnosed with diabetes ($4205 ±2596), asthma ($3307 ±2542), or COPD ($3779 ±2529) than for those without these conditions. MBS costs for people with asthma or COPD that had not yet been diagnosed were also significantly higher than for those without these conditions, although this was inconsistent across financial years. Conclusions: Diabetes, asthma, and COPD are costly conditions in terms of health service use. Costs associated with undiagnosed asthma and COPD are similar to their diagnosed states. Prevention of progression along each chronic disease continuum is likely to reduce costs.