Serum creatinine and drug concentrations were measured at 0, 1, 2, 3, 6, 9 and 12 months in 30 consecutive patients started on amiodarone. In 28 of these patients with no obvious cause for altered renal function, mean serum creatinine increased to 11% above baseline (P < 0.005). Rising creatinine concentrations correlated with amiodarone concentrations (y = 93.9 + 8.6x, r = 0.51, P < 0.0001). When assessing elevation of serum creatinine in a patient receiving amiodarone, physicians should be aware that it may be related to this drug.