While agreement is growing that mental illness burdens the North American economy, how it impacts productivity--particularly compared to physical illness--is unclear. Hypothesizing that lost work days are only the tip of the iceberg, we also examined the association of mental and chronic physical illness with partial work days and days requiring extra effort to function. Data from 4225 employed individuals, aged 18-54, were analyzed. These were a subset of respondents to the Ontario Health Survey's Mental Health Supplement, a 1990/91 epidemiologic survey of households across Ontario, Canada. Psychiatric disorder was assessed using the University of Michigan' modification of WHO's Composite International Diagnostic Interview (UM-CIDI). Similar to US reports, professional/managerial groups had lower rates of affective and anxiety disorders and fewer disability days compared to the rest of the workforce. However, no single occupational group was consistently at greater risk for either physical or psychiatric problems. Even after accounting for sociodemographic characteristics and work conditions, mental and physical status had clear, but different, impacts on productivity. Physical conditions alone had a fairly constant effect across all types of disability days and were the largest contributor to total work day loss. They also significantly impacted partial and extra effort days but were far less important than conditions involving a mental disorder. Respondents with mental health problems, either alone or in combination with physical illnesses, appeared more likely to go to work but to require greater effort to function. WHO projects that mental illness will become the second most important cause of global disease burden in the next century. Our findings suggest that among working individuals, it affects productivity more subtly than does physical illness. However, with an estimated eight percent of Ontario's workforce experiencing more than two months annually of decreased productivity, it still incurs significant social and economic costs.