This study examined the efficacy of a brief written intervention for primary care patients, designed to increase their level of participation in the consultation. Patients given the intervention leaflet (N = 59) were compared with those given a control leaflet (N = 61) on various consultation process and outcome measures. Psychological and sociodemographic data were also obtained to determine whether these influenced the effects of the intervention. The results showed that the intervention group had significantly longer consultations and asked more questions than the controls. Younger patients and those from social classes 1 and 2 were more likely to benefit from the intervention, but locus of control and self-efficacy scores were not particularly helpful in predicting outcomes. No differences in patient satisfaction were found nor were any negative effects on the doctor observed. A number of explanations are explored and some directions for future research are discussed.