Bird fancier's lung, the most common form of extrinsic allergic alveolitis in Britain, can be a difficult diagnostic problem. The symptoms are non-specific, often insidious in onset and frequently misdiagnosed as influenza or a viral or bacterial pneumonia. Frequently there is a delay in eliciting the history of exposure to the antigen. The chest radiograph is often less impressive than the clinical presentation and may be normal despite severe symptoms, impaired respiratory function and florid pathological changes. We present three cases demonstrating these diagnostic problem. In two cases, high resolution computed tomography demonstrated the typical 'ground glass' opacification seen in active alveolitis. This allowed targeting of transbronchial biopsies which revealed an inflammatory infiltrate of the interstitium with granuloma formation and inflammatory cells in some alveoli. The problems in diagnosis and the potential role of high resolution computed tomography are discussed.