We report on the case of a 86-year-old male patient with a rapidly growing nodule within the right lobe of the thyroid gland, which after hemithyroidectomy, turned out to be a mucosa-associated lymphoid tissue (MALT) lymphoma of the thyroid gland. In addition, Hashimoto's thyroiditis was reported in the thyroid tissue adjacent to the MALT lymphoma. During follow-up a second nodule emerged within the left lobe and, because of evidence of MALT lymphoma recurrence, F-18-FDG-PET was performed. F-18-FDG-PET imaged a clearly increased accumulation within the whole left lobe and isthmus. Thus, no differences in the degree of hypermetabolism could be imaged between the nodule and the adjacent thyroid tissue. To our knowledge, this is the first report about F-18-FDG-PET in a patient with MALT lymphoma of the thyroid. Literature search revealed only a few cases of MALT lymphomas in locations other than the thyroid gland that were studied with F-18-FDG-PET. In no case was F-18 FDG accumulation seen in the MALT lesions. However, clear F-18 FDG accumulation was reported in some patients with Hashimoto's thyroiditis. It is concluded that the intensive F-18-FDG accumulation within the whole left lobe and isthmus of the presented case was due to the coexisting Hashimoto's thyroiditis. Consequently, F-18-FDG-PET imaging does not seem to be indicated in a patient with MALT lymphoma and known Hashimoto's thyroiditis in order to evaluate the status of the MALT lymphoma.