The extent to which the length of the membrane-spanning part of intrinsic membrane proteins matches the hydrophobic thickness of the lipid bilayer may be an important factor in determining membrane structure and function. To gain insight into the consequences of hydrophobic mismatch on a molecular level, we have carried out systematic studies on well-defined peptide-lipid complexes. As model peptides we have chosen gramicidin A and a series of artificial hydrophobic alpha-helical transmembrane peptides that resemble the gramicidin channel. These peptides consist of a hydrophobic stretch of alternating leucine and alanine residues with variable length, flanked by tryptophan residues. Using wide-line NMR techniques, we have investigated the interaction of these peptides with the bilayer-forming diacyl phosphatidylcholines and with phospholipids which by themselves have a tendency to form non-bilayer structures. We have shown that hydrophobic mismatch leads to systematic changes of the bilayer thickness and that it can even change the macroscopic organization of the lipids. The type of lipid organization induced by the peptides and the efficiency of the various processes depend on the properties of the lipids and on the precise extent of mismatch.