The concentrations of organochlorine pesticides in serum have been determined by GC and electron capture detection. Studies were performed in 7 nondialyzed, 10 dialyzed and 6 healthy persons. Only hexachlorobenzene (HCB) and 1,1-di(4-chlorophenyl)-2,2-dichloroethene (p,p'-DDE) were consistently present. No interference by other pesticides or polychlorinated biphenyls was found by checking with GC-MS. For the nondialyzed uremic patients the average HCB concentration was 16.2 nmol/l (sigma = 6.7, n = 7), and the p,p'-DDE level 26.4 nmol/l (sigma = 31.4, n = 7). For the dialyzed uremic patients the average HCB level was 15.5 nmol/l (sigma = 11.2, n = 10) before dialysis and 17.2 nmol/l (sigma = 14.4, n = 8) after dialysis, the concentration of p,p'-DDE was 27.0 nmol/l (sigma = 38.4, n = 10) before dialysis and 28.0 nmol/l (sigma = 37.4, n = 8) after dialysis. For the healthy persons the average concentration of HCB was 7.7 nmol/l (sigma = 1.8, n = 6) and the concentration of p,p'-DDE was 20.1 nmol/l (sigma = 10.4, n = 6). HCB concentrations were significantly higher in serum of dialyzed and nondialyzed uremic patients than in controls (Wilcoxon's test).