Human serum albumin (HSA) is the most abundant protein in the circulatory system. Its principal function is to transport fatty acids, but it is also capable of binding a great variety of metabolites and drugs. Despite intensive efforts, the detailed structural basis of fatty acid binding to HSA has remained elusive. We have now determined the crystal structure of HSA complexed with five molecules of myristate at 2.5 A resolution. The fatty acid molecules bind in long, hydrophobic pockets capped by polar side chains, many of which are basic. These pockets are distributed asymmetrically throughout the HSA molecule, despite its symmetrical repeating domain structure.