A cytotoxin active on Vero cells, less active on hela cells, and inactive on WI38 cells (Vero toxin [VT]) was detected in stool isolates of Escherichia coli from 8 of 15 sporadic cases of haemolytic uraemic syndrome (HUS). Stools from 5 of these 8 patients were examined for faecal VT activity, and all were positive. Of the 7 of 15 patients who did not have VT ⁺ E. coli, 2 were positive for faecal VT, and a third (patient K) had strong serological evidence of VT⁺ E. coli infection. 2 HUS patients, including patient F, had siblings with uncomplicated diarrhoea who had both VT⁺ E. coli in the stools and faecal VT activity. Thus 11 of 15 (73%) of the HUS cases had evidence of infection by VT⁺ E. coli, suggesting that there was an association between these organisms and these cases of HUS. The clustering of 2 HUS patients and their siblings with VT⁺ E. coli accords with this suggestion, as do the rising titres of VT-neutralising antibody in 2 HUS patients.