We report a case of benign symmetric lipomatosis with hypothyroidism. Functional abnormalities and distribution of lipomas in benign symmetric lipomatosis suggest that the lipomas in this disorder may represent brown adipose tissue. In conditions where mRNAs of uncoupling protein, which is believed to be unique for brown adipose tissue mitochondria, were detected in one microgram of poly (A+) rat brown fat RNA, no signal at all was found in the lipomatous tissue, suggesting that the masses of benign symmetric lipomatosis are not functional brown adipose tissue.