Respiratory symptoms are the most common cause of general practitioner (GP) consultation, and hospital-based specialists are often called on to provide management guidelines, particularly in the area of antibiotic prescribing. The present authors have assessed factors associated with antibiotic use by 115 GPs when managing 1089 adults with an acute lower respiratory tract illness, including cough. They prescribed antibiotics to three-quarters of patients, but felt antibiotics to be definitely indicated in less than one-third of these cases and not needed in one-fifth. Univariate analysis revealed that antibiotics were prescribed more frequently by older GPs for older patients in the presence of underlying disease, discoloured sputum, shortness of breath, wheeze, fever, signs on chest examination, and 'other factors'. Multivariate logistic regression confirmed an independent effect for all these findings except for the presence of underlying disease, shortness of breath and wheeze. 'Other factors' included patient 'pressure' and social factors, and GP work pressure or prior experience with the patient. These factors were an important influence on prescribing, especially if the GP felt an antibiotic was not indicated. Amoxycillin was the first choice (58% of total) except where the patient had recently received antibiotics for the same illness. Broader spectrum antibiotics were used more commonly in patients with chronic lung disease, discoloured sputum, chest signs on examination and where the GP felt antibiotics were indicated. However, these antibiotics were also prescribed to 14% of previously well patients. General practitioners used a wide variety of terms to describe the illness with little consistency or structure. The decision concerning the use and choice of antibiotics and the confidence with which the GP makes that decision is a complex interaction between patient, doctor and disease, being affected not only by clinical features but also by the social and psychological elements of the presenting problem. Such issues need to be appreciated by hospital specialists when called on to advise on developing relevant guidelines for primary care.