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 x 10(-6) cases 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 correlate significantly with survival.