In this study we estimated the costs of back pain to society in The Netherlands in 1991 to be 1.7% of the GNP. The results also show that musculoskeletal diseases are the fifth most expensive disease category regarding hospital care, and the most expensive regarding work absenteeism and disablement. One-third of the hospital care costs and one-half of the costs of absenteeism and disablement due to musculoskeletal disease were due to back pain. The total direct medical costs of back pain were estimated at US$367.6 million. The total costs of hospital care due to back pain constituted the largest part of the direct medical costs and were estimated at US$200 million. The mean costs of hospital care for back pain per case were US$3856 for an inpatient and US$199 for an outpatient. The total indirect costs of back pain for the entire labour force in The Netherlands in 1991 were estimated at US$4.6 billion; US$3.1 billion was due to absenteeism and US$1.5 billion to disablement. The mean costs per case of absenteeism and disablement due to back pain were US$4622 and US$9493, respectively. The indirect costs constituted 93% of the total costs of back pain, the direct medical costs contributed only 7%. It is therefore concluded that back pain is not only a major medical problem but also a major economical problem.