STUDY OBJECTIVE--The aim of the study was to consider possible changes in the clinical activities of general practitioners whose patients are registered in a breast cancer screening programme. DESIGN--The study was a survey based on completion of forms recording breast consultations carried out by participating general practitioners during a four week period. SETTING--One of three intervention centres and one of three comparison centres in the national trial of early detection of breast cancer was selected. The intervention centre was in Guildford; the comparison centre in Stoke on Trent. PARTICIPANTS--The participants were general practitioners in the selected centres. In Guildford, 64 of 99 general practitioners approached took part (65%); in Stoke on Trent, 81 of 177 took part (46%). The proportion of male and female participants in the two centres was similar. Doctors in Stoke on Trent were older and worked in smaller practices than in Guildford. RESULTS--A comparison of workloads showed that in the screening centre there was less demand for doctor consultations from those in the screened age group, but those excluded from screening made more use of the general practitioners' services. A difference in referral practice was also apparent, with doctors in the screening centre referring more frequently for specialist advice. CONCLUSIONS--The evidence suggests that no significant change in the overall use of general practice resources can be expected with the introduction of national screening, but there may be greater pressure on assessment services.