The structure of the title compound (systematic name: 2-{2-[1-(4-chlorobenzoyl)-5-methoxy-2-methyl-1H-indol-3-yl]acetoxy}acetic acid monohydrate, C(21)H(18)ClNO(6).H(2)O) contains one-dimensional infinite hydrogen-bonded chains. Molecules of acemetacin, a nonsteroidal anti-inflammatory drug, and water are linked by three independent O-H...O bonds. The central unit in each chain consists of a sequence of alternating centrosymmetric R(4)(4)(12) and R(4)(4)(18) rings, which are edge-fused. Each ring links two acemetacin and two water molecules. A comparison with seven related structures reveals that molecules of acemetacin, indomethacin and their analogues can adopt two principal conformations.