Assessment of cognitive and emotional variables in 19 females with hyperthyroidism was made pretreatment, at 3 weeks, and after euthyroidism was established. A matched group of normal controls was similarly tested. Group differences on cognitive measures did not reach statistical significance, but cognitive deficits and symptoms of emotional disorder were significantly associated with the severity of thyroid toxicity previous to treatment. Measures of cognitive function and personality features moved towards control group values as euthyroidism was established. The implication of these findings is discussed in the context of a review of previous literature. The observed cognitive disturbance and emotional distress appear to be reflections of thyroid toxicity.