The nephrotic syndrome in Nigerian children is known to be largely associated with the endemicity of quartan malaria. Routine thyroid function studies were carried out on 24 children with clinical and biochemical evidence of the nephrotic syndrome. The children, aged four to 14 years, were all in the active phase of their disease, presenting with facial and pedal oedema and ascites. There was severe hypoalbuninaemia [mean (S.E.); 19.2 (1.1) g/l], hypercholesterolaemia; 10.5 (1.0) mmol/l and severe albuminuria ranging from 1 to 10 g/l. There was no clinical evidence of thyroid disease. The results of thyroid function tests in these children were compared with those of 181 apparently healthy children of the same age range. The mean total serum thyroxine levels (S.E.) were 118.3 (2.6) and 50.0 (6.4) nmol/l in controls and patients, respectively; T3 resin uptake values were 29.8 (0.2)% and 33.1 (1.2)%; the free thyroxine index (FTI) was 34.7 (0.8) and 16.7 (1.9) while thyrotropin (TSH) levels were 4.8 (0.2) and 10.6 (1.0) mU/l (IRP. MRC 68/38), respectively. The findings of low levels of thyroxine and FTI in association with high levels of TSH suggest that a state of primary hypothyroidism exists in these nephrotic children.