Dear Sir, Brown fat is mainly found in infants and children and gradually disappears from most sites of the body with increasing age. During adulthood, deposits of brown fat persist around kidneys, adrenal glands, and aorta and within the mediastinum and neck. Tumors composed of brown fat (hibernomas) are rare in breast [1, 2, 4, 5]. A rare case of hamartoma containing brown fat has been reported by Garijo et al. [5]. Adenohibernoma, hibernoma-containing glands, is also a very rare lesion as one case only has been described until now [2]. Here, we are reporting a second case. The patient, a 57-year-old lady, underwent excisional biopsy 2 cm across due to right breast lesion detected mammographically. The family history was unremarkable. Histologically, in addition to tubular carcinoma, an incidental lesion was detected nearby which had not been detected on mammograms. This latter lesion was well circumscribed and measuring 3 mm. It was formed partly by a fibroepithelial proliferation reminiscent of fibroadenoma with myxoid stroma (Fig. 1). Nuclear atypia and epithelial proliferation were not present. The remaining part of the nodular lesion showed dilated glands immersed within fat tissue, which showed uniform, normochromatic, and centrally located nuclei with vacuolated granular eosinophilic cytoplasms consistent with brown fat (Fig. 2). Accordingly, the vacuolated granular brown fat cells were positive with avidin–biotin complex peroxidase for S-100 protein (Dako, polyclonal, 1:1500; Fig. 3) and negative for wide spectrum keratin (Dako, MNF116, 1:200) and cluster of differentiation (CD)68 (Neomarkers, monoclonal, MAC387, 1:200). There was no residual lesion within the re-excision specimen which consisted of fibrofatty tissue measuring 4 cm. Seven lymph nodes from lower axillary level were reactive. There is no evidence of disease after a 6-year follow-up. Adenolipomas are lesions composed of glands immersed in adult classical adipous tissue and are closely associated with hamartomas. Both lesions are fibroglandular and are well circumscribed [3] and the amount of fat is by definition conspicuous in adenolipomas compared with hamartomas. Hamartomas are lesions that result from faulty development in an organ, composed of an abnormal mixture of normally occurring tissue elements or an abnormal proportion of a single element normally present at that site. Brown fat is not a normal constituent of adult breast tissue and in this respect adenohibernomas cannot be regarded as hamartomas. Damiani and Panarelli [2] suggested a hamartomatous origin from remnants of fetal brown adipose tissue or, alternatively, the possibility of de novo mixed proliferation showing brown fat differentiation in its mesenchymal component for adenohibernomas. In addition, the focus that the granular cells are associated to a fibroadenoma is a further proof that the adenohibernoma is not a hamartoma. This situation was favored by Fisher et al. [3] who believed that fibroadenomas and hamartomas were Virchows Arch (2008) 452:351–352 DOI 10.1007/s00428-007-0553-x