The spatial coherence of free-electron laser radiation inthe water window spectral range was studied, using the third harmonic(l3rd = 2.66 nm) of DESY’s Free-electron LASer in Hamburg (FLASH).Coherent single pulse diffraction patterns of 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC) multilamellar lipid stacks have been recorded. Theintensity histogram of the speckle pattern around the first lamellar Braggpeak, corresponding to the d = 5 nm periodicity of the stack, reveals anaverage number of transverse modes of ¯M = 3.0 of the 3rd harmonic. Usingthe lipid stack as a ’monochromator’, pulse-to-pulse fluctuations in the thirdharmonic l3rd have been determined to be 0.033 nm.