58 paediatric patients with pneumonia, in whom an etiological agent had been isolated, were reviewed. The patients were designated to have either viral or bacterial pneumonia on the basis of proposed clinical and radiological criteria. These presumed diagnoses were then compared to the microbiologically proven diagnosis. When clinical features suggested a bacterial infection the chance of isolating a bacteria as opposed to a virus was 18%. Was radiological features suggested a bacterial infection the chance of isolating a bacteria as opposed to a virus was 30%. Thus the commonest cause of "bacterial" clinical and radiological features is a viral infection and the proposed criteria do not allow differentiation of bacterial from viral pneumonia.