Thyroid function was evaluated in 16 patients with end-stage renal failure, 8 treated by maintenance haemodialysis (HD) and 8 by continuous ambulatory peritoneal dialysis (CAPD), and compared with 8 healthy subjects. Serum total thyroxine (T<sub>4</sub>) and triiodothyronine (T<sub>3</sub>) concentrations were low in both groups (p < 0.01) with T<sub>3</sub> significantly lower in the CAPD than the HD group (p < 0.01). Serum-free thyroxine (FT<sub>4</sub>) concentrations were low in CAPD patients with respect to normal (p < 0.0l). Basal thyroid-stimulating hormone (TSH) concentrations did not differ significantly from normal but the TSH responses to thyrotrophin-releasing hormone were depressed in both groups. While CAPD may improve control of some biochemical and haematological parameters as compared with HD, it does not have a beneficial effect on thyroid function.