From 1980 to 1990 4,229 consecutive euthyroid patients with thyroid nodule (73% with single and 27% with multiple nodules) were examined by FNA cytology for preoperative selection. One thousand four hundred and eight of these patients (33%) had nodules also suitable for evaluation by large needle biopsy histology (Aspiration Needle Biopsy, ANB). No significant complications occurred following ANB. The proportion of inadequate specimens was 25% for ANB and 15% for FNA, however a definite diagnosis was obtained with ANB in 62 patients with inadequate FNA finding. Diagnostic sensitivity was higher (93%) for FNA than for ANB whereas specificity was better (82%) for ANB diagnoses. Nonetheless ANB contributed to the increase of overall sensitivity as four of all the malignant nodules diagnosed as benign by FNA were correctly identified by ANB. Analysis of the postoperative results of 102 nodules with FNA and ANB finding of benign nodule or of suspected cancer showed that the addition to the same FNA finding (benign nodule or suspected cancer) of a different ANB diagnosis (suspected cancer or benign nodule) greatly changed the probability of finding a malignant nodule at postoperative histology. ANB was also useful in showing a macrofollicular component in 52% of 150 nodules diagnosed by FNA as pure microfollicular nodules.(ABSTRACT TRUNCATED AT 250 WORDS)