During 1 September 1981 to August 1982 aspiration cytology was carried out in all isolated thyroid swellings referred to the Aberdeen Thyroid Clinic: cytological findings were not disclosed, did not influence management, and were compared retrospectively with the histological diagnosis. In a total of 70 swellings sensitivity for the detection of neoplasia was 86% and overall accuracy 92%; the positive predictive value was 80% and negative predictive value 96%. During the second year (1 September 1982 to 31 August 1983), when cytological findings were used to influence management, the frequency of operation for isolated thyroid swellings decreased by 25% and the proportion of operations for neoplasia increased from 31% to 50%. In terms of bed occupancy the potentially avoidable surgical workload for benign disease was reduced by 34%. Aspiration cytology, carried out at the first clinic attendance, makes a sound basis for selective surgery and leads to economy in the management of isolated thyroid swellings.