To determine the prevalence of giardiasis in Gambian children with chronic diarrhoea and to assess their response to treatment, 31 children with chronic diarrhoea and malnutrition were investigated for giardiasis using a combination of serology (specific antigiardia IgM antibody) and microscopy of faeces and jejunal biopsy specimens. Fourteen of 31 children with chronic diarrhoea had giardiasis compared with only four of 33 healthy age and sex matched control children. Four of 15 malnourished children without diarrhoea were giardia positive. Twenty-three children with chronic diarrhoea were reinvestigated after treatment with metronidazole; giardia was found in 11 of them. These results show that giardia is highly prevalent in children with chronic diarrhoea and malnutrition and that the infection does not respond to standard therapeutic measures.