A prospective study was carried out to compare the effectiveness of 4 commonly employed methods of bowel preparation before colorectal surgery: mechanical, mechanical with an antibiotic against aerobes (neomycin), mechanical with another antibiotic against anaerobes (metronidazole), and finally mechanical with a combination of neomycin and metronidazole. The use of mechanical preparation as the sole method was discontinued half-way through the study for ethical reasons. 53 bowel preparations were carried out on 31 patients, 7 adults and 24 children with mainly Hirschsprung's disease and imperforate anus. The incidence of postoperative infections ranged from 40 to 46% in the first 3 groups, in contrast to only 6% in the fourth group (p less than 0.05). The majority of the organisms isolated were gram-negative bacteria. The results show that a combination of mechanical bowel washout with oral neomycin and metronidazole is a most effective method of bowel preparation before colorectal surgery.