Plasma factors influencing vascular PGI2-like activity (PSA) were studied in 45 patients with IgA nephropathy, 18 with Henoch-Schönlein purpura, including 8 children with nephrotic syndrome, and 41 controls. The results were compared with the levels of plasma high-density lipoprotein (HDL), low-density lipoprotein (LDL) and fatty acid components of plasma phospholipids. The plasma of 38 of 45 patients with IgA nephropathy and 14 with Henoch-Schönlein purpura showed a diminished ability or no ability to support PSA. Twenty-three patients with IgA nephropathy and 10 with Henoch-Schönlein purpura exhibited an inhibitory activity against PGI2 production. The plasma HDL level was lower, while the LDL level and the LDL/HDL ratio were significantly higher in IgA nephropathy and Henoch-Schönlein purpura cases than in the controls. A high LDL/HDL ratio was associated with a low plasma PSA. The levels of arachidonic acid and its precursor were not lower in the plasma of patients than in the controls. The decreased PGI2 synthesis may play an important role in the pathogenesis of IgA nephropathy and Henoch-Schönlein purpura, but it can not be explained by reduced PG precursors. LDL may have an inhibitory, and HDL a protective effect on PGI2 synthesis.