OBJECTIVE--To examine the factors influencing quality of informed consent. DESIGN--Prospective study comprising interviews with patients and patients' completing standard questionnaires. SETTING--Academic surgical unit of large teaching hospital. PATIENTS--265 patients undergoing intrathoracic, intraperitoneal, and vascular surgical procedures. Of these patients, 192 have been followed up for six months. MAIN OUTCOME MEASURES--Patients' recall of information at various points in the study; this score was compared by age, provision of written information, cognitive function, intelligence quotient (IQ), mood state and personality traits, and health locus of control. RESULTS--The patients were best informed immediately after signing the consent form and from then on recall of information deteriorated. A total of 172/250 (69%) patients admitted to not reading the consent form before signing it. Old age adversely affected recall of information at all assessment points. Impaired cognitive function reduced information recall only during the stay in hospital. Patients with above average IQs handled information better than those with a lower IQ except immediately after the signing of consent forms. Patients with an internal locus of health control (that is, those who believed their health to be in their own control) were better informed than those with an external locus of health control. Operation information cards improved recall only on the day of discharge. CONCLUSION--Elderly patients and patients with below average IQ, impaired cognitive functions, and an external locus of control have poor information recall. Written information may be more useful if given before admission to hospital.