Prothrombin ratios were measured 13-16 days after treatment in 148 subjects from Sierra Leone taking part in a double-blind placebo-controlled trial of ivermectin. Prolonged prothrombin ratios were observed more frequently in the ivermectin group, although this difference was not significant and no patients suffered bleeding complications. Further investigation of these patients failed to reveal any abnormality of liver function, although factor VII and II levels were reduced in most affected individuals, suggesting interference with vitamin K metabolism. Ivermectin has a minimal effect on coagulation and concern about mass treatment for this reason appears to be unjustified.