During a 9‐year‐period, 50 cases of primary non‐Hodgkin's lymphoma (NHL) of the thyroid gland were reported to a population‐based lymphoma registry covering western Denmark giving an incidence of 2.06×10−6cases per year. The male:female ratio was 1:4, and the mean age was 72.8 years for women and 62.8 years for men. On histomorphological reclassification 83% of the cases showed a high grade and 17% a low grade morphology. 98% had a B‐phenotype and 2% a T‐phenotype. In at least 33 of the cases, primary NHL of the thyroid gland was preceded by Hashimoto's thyroiditis and at least 25 of the patients had a high grade NHL which was transformed from Hashimoto's thyroiditis through a low grade B‐cell lymphoma of MALT type. The most frequent presenting symptoms were goitre (100%), hoarseness (57%), stridor/dyspnoea (55%) and dysphagia (45%); thirty‐six percent of the patients were hypothyroid at the time of diagnosis. Seventy‐six percent of the patients had localized disease (stages 1–2) and 24% had disseminated lymphoma (stages 3–4). Five year survival was 34.5% and 5 year cause‐specific survival 46.2%. The following factors were associated with a poor prognosis: stage 3–4 disease, elevated S‐urate, presence of hoarseness and age>66 years. Morphological subtype did not correla