The crystal structure of ninhydrin, C9H604, has been determined using filtered Cu K~ radiation and a Nonius integrating Weissenberg camera. The crystals are monoclinic, P21, with a= 11.24, b= 6-06, c = 5-77/~, b'= 99.1 °, and Z= 2. The intensities of 849 independent reflections were measured with a densitometer. The structure was solved using Patterson and Fourier methods and refined by the anisotropic full-matrix least-squares method to an R value of 0.062. The molecules are connected in layers parallel to (100) by two types of hydrogen bonds. Each molecule is linked along c by two hydroxylcarbonyl bonds and about a screw axis in the b direction by two hydroxyl-hydroxyl bonds. The oxygenoxygen distance is 2.80/~ for both types. One carbonyl oxygen is not involved in the hydrogen bonding scheme.