The pathological changes observed in the lungs of 197 freshly dead to moderately decomposed harbour porpoises (Phocoena phocoena) stranded in England and Wales between October 1990 and December 1996 were reviewed. In 135 (69 per cent of the cases) macroscopic nematode infections of the bronchial tract with Pseudalius inflexus and Torynurus convolutus, either singly or in combination, were recorded, and 106 (54 per cent) also had P inflexus within the pulmonary blood vessels. All the macroscopically parasitised porpoises were adults or juveniles although two neonates had histological evidence of nematode infection. There were 62 cases of mild to severe, subacute to chronic bronchitis and bronchiolitis, 113 cases of mild to severe chronic granulomatous interstitial pneumonia, and 34 cases of mild to severe vasculitis or thrombovasculitis of pulmonary blood vessels attributable to these nematode infections. In 35 cases necropurulent or purulent (broncho)pneumonias were attributed either to secondary bacterial infections of the lungs or to septicaemias associated in seven cases with Streptococcus canis, in two cases with group B Salmonella species, in one case with Escherichia coli and in one case with Streptococcus lactis. The pulmonary lesions in 67 animals known or diagnosed to have been entrapped in fishing gear were non-specific and included persistent foam in the airways in 45 cases, diffuse congestion in 53, oedema in 50, and multifocal intra-alveolar haemorrhage in 33 cases. Seven cases of acute fibrinous or chronic fibrous pleuritis, seven cases of chronic necropurulent pneumonia associated with mycotic infections, four porpoises with traumatic lesions of the thorax and other parts of the body consistent with fatal attack from bottlenose dolphins (Tursiops truncatus), and one case of diffuse bronchointerstitial pneumonia associated with generalised morbillivirus infection were also recorded.