The work factors involved in physicians' mental workload were analysed. The study settings were 116 physicians (general practitioners, paediatricians and residents in family programmes working in out-patient clinics) (Primary Health Care Centres) in Spain. The questionnaire used measured the mental workload and the work activity over a period of 5 days. The mental workload covered seven dimensions: mental effort, physical effort, difficulty, priority, modify standards of visit, overlap, and stress. Its Cronbach's alpha coefficient was 0.80. The mental workload correlated positively with all variables measuring the workload activity. However, when the multiple regression analysis (stepwise met) was assessed, only the number of room visits, the referrals by nurse to physician, the room emergencies, the level of administrative tasks, the patients with complex pathologies, and the workload adequacy remained significant. The physicians' years of experience did not correlate with mental workload. The influence of diverse work activities in each of the mental workload dimensions was investigated. We conclude that the number of patient visits and organizational factors determine physicians' mental workload.