The authors of this paper assessed the effect of psychosocial and biomedical factors on the prescription rate for upper respiratory infections. Their study, using written clinical vignettes mailed to a random sample of Ontario family practitioners, achieved a 69% response rate. Family practitioners most often used physical signs and symptoms in forming their decision to prescribe antibiotics. Psychosocial data, when present in the vignettes, significantly modified the prescription rate, often overriding the biomedical factors alone. The study is another step in exploring a complex decision-making process.