Serial changes in thyroid hormone levels are described in two patients in whom hyperthyroidism was associated with transient non-thyroidal illness. In a 74-year-old woman with mild hyperthyroidism, two episodes of cholecystitis were associated with subnormal concentrations of serum T3 and increased concentrations of serum rT3; T3 became elevated during recovery, associated with a simultaneous fall in rT3. The TSH response to TRH was undetectable on three occasions. A cholecystectomy was performed after preparation with Lugol's iodine and subsequent tests showed evolution through T3 toxicosis to classical hyperthyroidism. In the second case, symptoms and signs of classical hyperthyroidism were noted during an undiagnosed illness characterized by severe abdominal pain and fever. Six days after the onset of this illness, an elevated level of serum T4 was associated with a normal total T3 concentration and increased concentration of rT3. After resolution of abdominal symptoms, serum T3 was markedly increased, associated with persistent T4 and rT3 excess. These findings indicate that the changes in T3 and reverse T3 described in non-thyroidal illness also occur in hyperthyroid patients, and suggest that the fall in T3 may be of sufficient magnitude to make T3 measurement diagnostically unreliable in the presence of non-thyroidal illness.