Non-compliance results in several undesired consequences. Admissions due to non-compliance have been estimated to account for up to 10.5% of all admissions to hospital. There seems to be very little data about compliance in Saudi Arabia. The present study addressed the problem of non-compliance with short-term antibiotic therapy in patients attending Primary Health Centres (PHC). The data were collected from five different centres selected randomly from the 53 centres in the Riyadh area, Saudi Arabia. A five-part questionnaire was designed and used to collect data. Different parts were required to be completed by patient, doctor, pharmacist and social worker. At the end of the study period 414 questionnaires were suitable for evaluation. Paediatric patients (< 15 years old) constituted 65.9% of the sample. Compliance was noted in 67.8%. Those who missed three doses or less and more than three doses were 22.7% and 9.4% respectively. Factors which appeared to enhance patient compliance were: parental involvement (p < 0.001), unemployment (p < 0.01), absence of psychiatric illness (p < 0.02) and early improvement of symptoms (p < 0.05). Reasons most frequently mentioned by patients for non-compliance were: rapid improvement of symptoms, bitter taste of drug(s), forgetfulness and frequent dosing. These reasons accounted for 73.7% of reasons for non-compliance. Our findings suggest that approximately two thirds of patients were compliant with their medications. It is also worth noting that approximately three quarters of patients were not compliant for reasons which could be minimised or removed by good patient counselling and effective communication with patients.(ABSTRACT TRUNCATED AT 250 WORDS)