Mental illness affects a large number of people in the world, seriously impairing their quality of life and resulting in high socioeconomic costs for health care systems and society. Our aim is to estimate the socioeconomic impact of mental illness in Spain for the year 2002, including health care resources, informal care and loss of labour productivity. A prevalence-based approach was used to estimate direct medical costs, direct non-medical costs, and loss of labour productivity. The total costs of mental illness have been estimated at 7,019 million euros. Direct medical costs represented 39.6% of the total costs and 7.3% of total public healthcare expenditure in Spain. Informal care costs represented 17.7% of the total costs. Loss of labour productivity accounted for 42.7% of total costs. In conclusion, the costs of mental illness in Spain make a considerable economic impact from a societal perspective.