Thyroid aspiration biopsy is a valuable complementary method for the examination of an enlarged thyroid gland--especially of cold nodules. Undifferentiated tumours and papillary carcinomas can be recognized with great accuracy. The inability to achieve a precise distinction between atypical follicular adenoma and highly-differentiated follicular carcinoma leads to false-positive or false-negative findings--depending on the strategy of cytological evaluation used. The selective value of the method can be preserved only by accepting false-positive cytological findings; false-negative findings are rare. Aspiration biopsy and cytological examination of the thyroid gland, therefore, can make a valuable contribution towards obtaining a precise indication for the surgical treatment of cold nodules of the thyroid.