ABSTRACT Aims The objective of this study was to estimate the social cost of the consumption of illegal drugs in Spain. Design We performed a cost-of-illness study, using a prevalence approximation and a societal perspective. The estimation of costs and consequences referred to 1997. As direct costs we included health-care costs, prevention, continuing education, research, administrative costs, non-governmental organizations and crime-related costs. As indirect costs we included lost productivity associated with mortality and the hospitalization of patients. Estimation of intangible costs was not included. Findings The minimum cost of illegal drug consumption in Spain is 88 800 million pesetas (PTA) (467 million dollars). Seventy-seven per cent of the costs correspond to direct costs. Of those, crime-related costs represent 18%, while the largest part corresponds to the health-care costs (50% of direct costs). From the perspective of the health-care system, the minimum cost of illegal drug consumption is 44 000 million PTA (231 million dollars). The cost of illegal drug consumption represents 0.07% of the Spanish GDP. This gross figure compares with 2250 million PTA (12.5 million dollars) invested in prevention programmes during the same year, and with 12 300 million PTA (68.3 million dollars) spent on specific programmes and resources for the drug addict population. Conclusions Although there are limitations intrinsic in this type of study and the estimations obtained in the present analysis are likely to be an underestimate of the real cost of this condition, we estimate that illegal drug consumption costs the Spanish economy at least 0.2% of GDP.