The capacity of leukocytes to produce prostacyclin (PGI2) from endogenous and from platelet-derived endoperoxides was tested in whole blood. During the acute phase of the hemolytic uremic syndrome (H.U.S.), the PGI2-production was lower than the controls, whereas the blood from children with chronic renal failure produced higher amounts. Production of PGI2 by blood from children 3/12 to 6 years after the acute phase of H.U.S. was normal, as was the case with blood from their parents. Furthermore, in two H.U.S.-patients studied serially, the decreased PGI2-production capacity normalized 2 1/2 months after the acute phase.