Thyroid gland scintigraphy was performed in 29 dogs with histologically confirmed thyroid tumors. Twenty dogs were female, and 9 were male. Median age was 10 years. Of the 29 dogs, 21 were initially examined because of cervical swelling or a cervical mass. Of the 29 tumors, 24 were thyroid adenocarcinomas, 1 was a C-cell carcinoma, 3 were undifferentiated carcinomas, and 1 was a thyroid adenoma. Serum triiodothyronine and thyroxine concentrations were determined in 25 dogs. Sixteen dogs were euthyroid, 6 were hyperthyroid, and 3 were hypothyroid. In all 29 dogs, results of scintigraphy were abnormal. The most common scintigraphic appearance (13 dogs) was a unilateral thyroid mass with increased radionuclide uptake, relative to that of the parotid salivary glands. There did not appear to be an association between distribution of radionuclide uptake and histologic diagnosis, although there appeared to be an association between distribution of uptake and histologic degree of capsular invasion. All 4 dogs with extensive capsular invasion and 11 of 17 dogs with limited capsular invasion had poorly circumscribed, heterogeneous uptake of pertechnetate by the tumor. All hyperthyroid dogs had intense uptake, and 5 of 6 hyperthyroid dogs had well-circumscribed, homogenous uptake. Scintigraphy did not appear to offer any additional benefit, compared with thoracic radiography, for detection of pulmonary metastases.