A case of acute-on-chronic renal failure in a 70-year-old woman with ischemic nephropathy and primary hypothyroidism is presented. Her renal function became progressively worse as the level of serum creatinine increased from 283 to 628 µmol/l (3.2–7.1 mg/dl) within 8 months. Her thyroid function had been normal before the exacerbation of renal failure, but it was markedly reduced with a marked elevation of serum thyroid-stimulating hormone. Thyroid hormone replacement therapy resulted in rapid improvement of the renal function to 159 µmol/l (1.8 mg/dl) of serum creatinine. The development of primary hypothyroidism seemed to worsen the already impaired renal function in this case. We suggest the assessment of thyroid function in patients with unexplained deterioration of renal failure.