Objective. This prospective study was conducted to examine the factors that affect swallowing during thyroid fine-needle aspiration biopsy (FNAB). Methods. Consecutive patients who were to undergo FNAB were approached for inclusion in the study. The depth and size of the nodule were measured in all patients. In a random fashion, patients were told not to swallow during the procedure, or they were not given any instructions regarding swallowing. Results. Regarding the 143 participants in the study, no significant differences in age (P=.07), diameter and depth of the thyroid nodules (P=.14; P=.46, respectively), or cytologic diagnosis of thyroid aspirates (P>.20) were found between the swallowing (n=50) and nonswallowing (n=93) patient groups. Swallowing was observed in 29 patients who were instructed not to swallow (36%) and in 21 patients who were not given any instructions regarding swallowing (34%; P=.95). The mean duration of the procedure for swallowing patients was 19.8 seconds, and it was 15.7 seconds for nonswallowing patients. The duration of the procedure was significantly shorter in patients who did not swallow (P=.001). More male patients swallowed than female patients (P=.003). Conclusions. Giving specific instructions about not swallowing did not make a difference regarding swallowing by patients during the FNAB procedure. More male patients swallowed than female patients. A longer duration of the FNAB procedure meant that the operator was more likely to encounter swallowing (and thus displacement of the thyroid) while doing the procedure. Completing the procedure quickly is the surest method to avoid a moving thyroid while performing FNAB.