Nalidixic acid (NA: 55 mg/kg daily for 5 days) is the recommended treatment for uncomplicated bacillary dysentery in areas where multidrug-resistant Shigella are prevalent. An open randomized comparison of this NA regimen with 2 doses of ofloxacin (total 15 mg/kg) was conducted in 1995/96 in 135 Vietnamese children with fever and bloody diarrhoea. Sixty-six children with a bacterial pathogen isolated were eligible for analysis. Of the 63 Shigella isolates, 39 (62%) were resistant to multiple antibiotics. Resolution times for fever and diarrhoea were similar in the 2 groups, but excretion time of stool pathogen was significantly longer in the NA recipients [median (range) days 1 (1-9) vs 1 (1-2), P = 0.001]. There were 9 (25%) treatment failures in the NA regimen and 3 (10%) in the ofloxacin group; P = 0.1. Two patients had NA-resistant Shigella flexneri. One of these isolates was selected during NA treatment. From a clinical and public health standpoint a 2-dose regimen of ofloxacin is preferable to nalidixic acid in the treatment of bacillary dysentery.