We have investigated the morphology, cytogenetics, and the fraction of mtDNA with the tRNA (Lys) A--> G (8344) mutation in three lipomas in a carrier of this mutation. The son of the patient had myoclonus epilepsy and ragged-red fibers syndrome. The fraction of mtDNA with the tRNA (Lys) mutation varied between 62% and 80% in cultured skin fibroblasts, lymphocytes, normal adipose tissue, and muscle. In the three lipomas the mean fraction of mutated mtDNA was 90%, 94%, and 94%. Ultrastructural examination of the lipomas revealed numerous mitochondria with changes such as electron-dense inclusions in some adipocytes. When considered cytogenetically, the lipomas were characterized by a mixture of karyotypically abnormal and normal cells. An identical del (6)(q24) was found in two tumors. The fraction of mutated mtDNA in cultured lipoma cells was the same as in the lipoma in situ, indicating that the cultured cells were representative of the primary tumor. These findings indicate that the lipomas have originated with a grossly normal stem line and subsequently have developed the 6q deletion. We conclude that the lipomas represent clonal growth of adipocytes with a high content of mtDNA with the tRNA (Lys) mutation. The tRNA (Lys) mutation may be either the direct or the indirect cause of pertubation of the maturation process of the adipocytes, leading to an increased risk of lipoma formation.