Fine-needle aspiration (FNA) with ultrasonography is considered a minimally invasive and safe procedure. Complications of it are infrequent and occur immediately or within a few days after FNA. Such complications may occur mainly in patients with underlying problems. We here report a rare case of thyroid cystic nodule infection occurring 1 month after FNA in an immunocompetent patient and serial sonographic findings in this patient. A 33-year-old woman with a cystic nodule including partially isoechoic solid areas on the right thyroid gland complained of difficulty swallowing and painful sensations in the right neck 1 month after FNA. On follow-up examination, the cystic nodule and perithyroidal soft tissue were suspicious of infection. The possibility of infection after FNA should be considered even if the patient is immunocompetent in order to prompt evaluation and immediate management with empirical antibiotic therapy to avoid life-threatening complications.