In the acute phase of Graves' thyrotoxicosis patients often have subjective cognitive complaints. Continuing controversy exists about the nature of these symptoms and whether they persist after treatment. This prospective study included 31 consecutively referred, newly diagnosed, and untreated patients with Graves' thyrotoxicosis. A control group of 34 individuals matched for age, education and premorbid intelligence was also included. At baseline all patients and control subjects were examined with psychiatric rating scales and a comprehensive neuropsychological battery. The effect of treatment on affective symptomatology was examined in the patient group after reaching euthyroidism and 1 year after treatment initiation. At initial examination patients had significantly higher scores on psychiatric rating scales as compared with controls, and the majority reported memory and concentration problems. No significant differences between the patient and the control group on neuropsychological test performances were found. Thyroid levels did not correlate with the neuropsychological test performances or psychiatric ratings. After reaching euthyroidism the level of affective symptoms (including reports of cognitive deficits) had decreased significantly, with further normalisation 1-year after treatment initiation. In conclusion, patients had subjective reports of cognitive deficits in the toxic phase of Graves' thyrotoxicosis but comprehensive neuropsychological testing revealed no cognitive impairment. Reports of cognitive dysfunction may reflect affective and somatic manifestations of thyrotoxicosis and in most patients these symptoms disappear after treatment of Graves' thyrotoxicosis.