In the title compound (systematic name 1-hy-droxy-1,2-dihydro-obacunoic acid 3,4-lactone monohydrate), C(26)H(32)O(8)·H(2)O, the dihedral angles between the planes of the ester groups and the furan plane are 43.06 (12) and 56.06 (7)°, while that between the furan plane and the keto group is 58.50 (9)°. The A/B, B/C and C/D ring junctions are all trans-fused. Inter-molecular O-H⋯O hydrogen bonds between the hy-droxy and carbonyl groups and the water mol-ecule give rise to a three-dimensional structure.