Background. While cross-sectional and longitudinal studies have consistently found depressive illness and disability to be related, understanding whether depression leads to subsequent onset of disability is limited.

Methods. In the context of the multi-centre international WHO Collaborative Study on Psychological Problems in General Health Care, we followed prospectively consulting non-elderly primary care patients who were essentially disability free at baseline but who differed in baseline depression status, comprising 1051 patients free of physical disability at baseline including 14% depression; 914 free of social disability including 9% depression. Depression status was assessed with the CIDI; patient-reported physical disability with the MOS physical functioning scale and social disability with the BDQ role functioning and number of disability days measures; investigator-rated social disability with the Occupational section of Groningen Social Disability schedule; and the treating physicians rated the severity of physical illness.

Results. In patients essentially disability free at baseline, depressive illness resulted in a 1·5-fold (at 3 months) and a 1·8-fold (at 12 months) increase in risk of onset of physical disability, after controlling for physical disease severity. Depressive illness also resulted in a 2·2-fold (at 3 months) and a 2·3-fold (at 12 months) increase in risk of onset of social disability, after controlling for physical disease severity, physical disability and onset of physical disability.

Conclusions. Among non-elderly primary care patients, depressive illness is associated with onset of physical disability and shows an even stronger association with onset of social disability.