In biopsies from spontaneously ruptured Achilles tendons a serum lipid and lipoprotein analysis was performed in fifty nine patients. In nine males considerable histological signs of lipid deposition were detectable. The tendon fibers were found to be partly desintegrated. Only in these cases LDL-cholesterol values were found to exceed the normal range indicative of type II hyperlipoproteinaemia (HLP). None of the fifty remaining patients presented biochemical or clinical signs of disorders of lipid metabolism. The observations demonstrate that a spontaneous rupture of Achilles tendom may indicate a primary type II HLP.