During an outbreak of Flavobacterium meningosepticum septicaemia in a neonatal intensive care unit 9 infants were treated with intravenous trimethoprim sulphamethoxazole. Bacteriological cure was achieved in 8 patients; one infant died of massive intraventricular haemorrhage on the first day of treatment. Apart from prolonged persistence of pre-existing thrombocytopenia there was no evidence of side effects. Trimethoprim sulphamethoxazole should be considered in the treatment of neonatal F meningosepticum sepsis in view of its activity against this organism, good penetration of the blood brain barrier, and the absence of serious side effects.