Between January 1980 and December 1988, 161 patients underwent thyroidectomy with intraoperative frozen section consultation after fine-needle aspiration (FNA) of a thyroid nodule. The FNA were insufficient in 15 instances (9%) and in error in 39 (24%). In 15 cases, the incorrect aspiration diagnosis could have led to excessive surgery and in ten cases to delayed therapy if it had been the only guide for therapy. The diagnosis was deferred to permanent section analysis in 30 (19%) frozen sections. Twenty-two errors (14% of cases) were made in the interpretation of frozen section material, and in an additional 15 patients (9%), the diagnosis suggested (but deferred at frozen section) was in error. In one patient, this error could have led to more extensive surgery than necessary; in 21 patients, the frozen section error could have led to undertreatment. When frozen section results were combined with those of FNA, no therapeutically important false-positive diagnoses were made. In five patients, the combination of both FNA and frozen section results would not have identified a carcinoma which, in three cases, was a small occult papillary carcinoma not found in the index nodule.