Fine-needle aspiration biopsy of palpable lesions of the thyroid is a diagnostic method routinely used in several medical centres, but still relatively new in others. Experience has shown that the technical steps involved in this biopsy procedure, that is, palpation, aspiration, smear making, fixation and staining, are critical for good results, and they are therefore reviewed and illustrated. Another prerequisite for the use of aspiration biopsy cytology is a specially interested cytopathologist who is familiar with the methodological aspects, as well as the interpretation of cells in aspirates. A description is given of the different cytological patterns in smears of fine-needle aspirates for clinical thyroid diseases, such as simple and toxic goitre, different types of thyroiditis (acute, subacute and chronic) as well as the various differentiated thyroid neoplasms (follicular, papillary, medullary), anaplastic carcinomas and lymphomas. The limits of cytological diagnosis are indicated and differential diagnostic aspects are discussed. diagnostic accuracy of aspiration biopsy cytology, with particular reference to tumour diagnosis, is high: no false positive diagnoses are made by experienced cytopathologists, whereas false negative tumour diagnosis can occur in about 10 per cent of cases. Such results are the reason for the special position of fine-needle aspiration biopsy among other methods in the diagnostic work-up of patients with thyroid disease.