This study aims to determine the growth pattern and in vitro susceptibility of clinical isolates of Blastocystis hominis to different concentrations of metronidazole, furazolidone and ciprofloxacin. Stool specimens from 25 consecutive patients with irritable bowel syndrome presenting to the gastroenterology department of Aga Khan University Hospital between January and May 2003 are examined by microscopy and cultured for B. hominis. Drug susceptibility assays are performed for metronidazole, furazolidone, and ciprofloxacin using final concentrations of 0.01 mg/mL and 0.1 mg/mL. The effect of the drugs is assessed after B. hominis culture for 48 h. With furazolidone and metronidazole, 68% (17/25) and 60% (15/25) of B. hominis isolates, respectively, failed to grow at drug concentrations of both 0.01 mg/mL and 0.1 mg/mL. However, ciprofloxacin failed to suppress growth completely at both concentrations. B. hominis resistance to furazolidone, metronidazole and ciprofloxacin at 0.01 mg/mL was 32% (8/25), 40% (10/25) and 100% (25/25), respectively. B. hominis isolates varied in their degree of susceptibility to the three drugs studied, being greater with furazolidone than with metronidazole, and complete resistance with ciprofloxacin.