A study of 239 patients compared free thyroxine (FT4) measurements made by equilibrium dialysis (ED) with measurements made using the Magic Lite FT4 chemiluminescence (CI) immunoassay (Ciba Corning Immunodiagnostics). Patient groups: 41 normals; 27 hyperthyroid; 29 hypothyroid; 37 sick euthyroid; 10 chronic renal failure (CRF) and 25 pregnant patients; 13 oestrogen; 10 heparin; 12 salicylate; and 9 dilantin-treated patients; 3 lipaemic; 5 haemolysed; 6 hyperbilirubinaemic patients; 6 low thyroid binding protein (TBP) and 6 high TBP level patients. The t w o assays gave comparable results in most groups. Both assays tended t o give elevated values in heparinized patients but FT4-ED results were more obviously affected. Pregnant patients and women on oral oestrogen had higher mean values with FT4-ED. In both assays the sick euthyroid and CRF patients had mean FT4 values similar t o healthy euthyroid patients; the range of values in sick euthyroid and CRF patients was similar in both assays but wider than in healthy euthyroid patients. A supplemental study of 81 unselected acutely ill patients using FT4-CI alone confirmed the wider range of values t o be anticipated in sick euthyroid patients.