Median concentrations (±standard errors of the median) of anaerobic bacterial CFU per gram of fecal sample in patients with grades 0 to I (□; n = 84) or grades II to IV (▩; n = 50) acute GVHD in the week before and during 5 weeks after allogeneic marrow transplantation. Median concentrations of anaerobic bacterial CFU in patients with grades 0 to I acute GVHD were below the detection threshold (10³ per gram of sample) in week 5 posttransplant. Differences between both patient groups were compared using repeated measures analysis of variance.22