Lanreotide is a therapeutic octapeptide used against acromegaly. It has been synthesized by Beaufour/Ipsen which discovered that a gel, formed by 30% of the peptide in pure water, when used as subcutaneous implant induces the sustained release of the peptide in the blood circulation during one month. The molecular and supramolecular organization of this unique peptidic gel has been determined and found to be related to the formation of nanotubular structures. These nanotubes are formed by the association of b-sheet filaments, the building block of which is a dimer of a small peptide. The evolution of the structures was studied as a function of temperature and concentration. The results allow us to draw the Lanreotide-water phase diagram and to propose a model for the self-association mechanisms of Lanreotide filaments into these nanotubes. In addition, the mechanism of formation of these structures, that exhibits similarities with the mechanism of amyloid fibril growth, can be used as a simple model to identify the driving forces leading to protein self association.