Fourteen children with acute post-streptococcal nephritis had daily measurements of urine fibrin degradation products (FDP) excretion during the acute phase of their illness. The amount of FDP excreted daily varied greatly but a general decrease occurred as other parameters showed resolution of the illness. Some children showed no increase in FDP output. We feel, however, this test may help in monitoring the progress of this illness in certain patients. Fibrin degradation products (FDP) are excreted in the urine in many forms of glomerulonephritis (Clarkson and others, 1971). The origin of the FDP is renal and not filtrated from the blood (Ekberg, Pandolfi, 1975). Clarkson further showed that in post-streptococcal glomerulonephritis there is a cessation of FDP excretion at the time of clinical recovery. We have studied a group of children with acute post-streptococcal nephritis in order to further assess the extent and possible value of daily FDP estimations in the management of patients during the acute phase of the disease.