The incidence of malignant tumors and leukemia was analyzed in 829 patients with chronic lymphocytic thyroiditis and in 829 individually age-matched and sex-matched patients with colloid goiter. Diagnoses were based on cytologic studies of specimens obtained by fine-needle aspiration biopsy. The patients were examined between 1959 and 1978 and were followed in the Swedish Cancer Register between 1959 and 1981. There was no increased risk for the total number of tumors in the thyroiditis group (53 observed vs. 52.7 expected) or in the colloid-goiter group (40 vs. 53.2, respectively; P not significant). There were six lung cancers in the thyroiditis group (2.9 expected, P not significant), and one in the group with colloid goiter. Patients with thyroiditis had an increased risk of myeloproliferative and lymphoproliferative neoplasms (12 observed vs. 3.0 expected, P less than 0.001). The risk of malignant thyroid lymphoma was greatly increased, with an estimated relative risk of 67 (4 observed vs. 0.06 expected, P less than 0.000001). There was no increased risk for any type of tumor among patients with colloid goiter.