23 patients undergoing marrow transplantation for leukemia or aplastic anemia were given cyclosporin A (CyA) for graft-versus-host disease (GvHD) prophylaxis. A radioimmunoassay was used to monitor CyA serum levels in 13 patients and whole blood levels in 10. Serum creatinine levels were recorded daily until day 30 and then weekly. The severity of acute GvHD was recorded daily for a total of 1,738 patient days. CyA dose was then correlated with CyA serum or blood levels, serum creatinine levels and severity of acute GvHD. The daily dose of administered CyA correlated with serum CyA levels (p = 0.001) but not with whole-blood CyA levels. The cumulative CyA dose correlated with serum creatinine. There was an inverse correlation between the daily CyA dose and the severity of acute GvHD (p = 0.05). On the other hand the total amount of CyA given within 10 days after bone marrow transplantation had no influence on the severity of acute GvHD developing after day 10. Serum and whole-blood levels did not correlate with severity of GvHD nor with creatinine levels. The results of this study point out the nephrotoxicity of CyA, and a low GvHD score with high doses of administered CyA, at least on a daily basis. Serum but not blood levels reflect the dose of CyA given, but are not correlated with nephrotoxicity or GvHD.