The pathological and metabolic characteristics, as well as the anatomical and functional evolution of lumbosacral lipomas, were studied in a series of 73 patients operated on between 1970 and 1983. The pathological study showed that they are mainly composed of adipocytes, but that they also possess fibrous tissue, vessels, and nerve fibers. Innervated muscle fibers, contracting under proper nerve stimulation, were found in several cases. Lipogenesis and lipolysis are the same in lumbosacral lipomas and in normal fat tissue. However, these lipomas can grow with the rest of the fatty pool. Moreover, spontaneous progressive worsening of the clinical status has been observed in 36% of the cases. Surgery is efficient and not harmful to the patient. Postoperative mortality was nil. Early postoperative worsening occurred in 2.7% of the cases. The 6% rate of late postsurgical deteriorations should be compared to the 36% to 56% rate found when patients are not operated on. Three different mechanisms are responsible for clinical worsening: compression or stretching of the cord, and cord injury on the posterior upper limit of the spinal defect. In each case, one mechanism is prevalent. Three different types of lumbosacral lipomas can thus be individualized. The surgical implications of these data are discussed. The necessity for early and systematic surgical treatment is pointed out.