Pathological investigations were carried out on 67 guillemots (Uria aalge) washed up on the Belgian coast between November 1993 and March 1994. Emaciation and acute haemorrhagic gastroenteropathy were observed in more than 70 per cent of the birds. There was no statistical relationship between the level of oil contamination and the severity of the lesions. Differences in bodyweight were accounted for by age, sex, emaciation, and acute haemorrhagic gastroenteropathy. The birds had a severe weight deficit but the concentrations of pollutants were below acutely toxic levels.