A possible interaction between fluvastatin, a new HMG CoA reductase inhibitor, and cyclosporin A (CsA) was studied in 16 stable renal transplant patients with dyslipoproteinemia despite dietary control. They entered a 12-week study period: 4-week baseline, 4-week placebo and 4-week fluvastatin treatment (20 mg daily). Weekly trough whole blood CsA concentrations were measured by monoclonal antibody immunoassay. Serial renal and liver function tests and creatine phosphokinase (CPK) were monitored. The mean trough levels of CsA during fluvastatin treatment were 115.6 micrograms/l compared with 114.3 micrograms/l and 113.3 micrograms/l in the baseline and placebo period, respectively. Apart from a small decrease in serum albumin level (from a mean of 38.6 g/l to 37.9 g/l), there were no significant changes in renal and liver functions or CPK levels. No adverse effects were reported. Fluvastatin (20 mg daily) may be used safely without extra monitoring of blood CsA concentrations in renal transplant patients.