Food deprivation is known to suppress antimicrobial defence under certain conditions. This potentially negative consequence of weight reduction treatment for obesity was studied in 11 obese patients who were treated with a mixed, balanced 600 kcal(2.5MJ)/day diet for 4 weeks. The mean body weight fell from 103 +/- 5 to 95 +/- 6 kg. The antimicrobial defence was studied before and during the diet period by analyses of polymorphonuclear granulocyte bactericidal capacity and adherence. No signs of impaired granulocyte function were found. The possibility remains that during prolonged treatment by semistarvation host defence may be impaired, especially in grossly obese patients whose antimicrobial defence may be already impaired before treatment.