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Rationale: Mesalazine and its prodrug sulfasalazine are both used for inflammatory bowel disease. Sulfasalazine has been associated with hematological side-effects such as aplastic and hemolytic anemia in patients, but also in fetuses after intrauterine exposure. To our knowledge, we describe the first case of a fetus with severe anemia, and subsequent hydrops, where this drug was found at concentrations in the fetus corresponding to those in the mother and most likely responsible for the fetal condition. Patient concerns: A uniparous woman was referred at 31 weeks of gestation due to a hydropic fetus with massive ascites and cardiomegaly. Diagnoses: The patient had Crohn's disease and was thus treated with 4 g mesalazine daily. The fetus had severe anemia with an initial hemoglobin level of 51 g/L. Interventions: The maternal medication was discontinued and four intrauterine erythrocyte transfusions were given during three weeks. Plasma samples were drawn from mother and fetus during cordocentesis for later analysis of mesalazine. Outcomes: A healthy baby was born after 37 full weeks of gestation. Plasma levels of mesalazine were non-conspicuous in neither mother nor fetus. The mesalazine half-life in the fetus (37 h) was half that of the mother (80 h), both considerably longer than previously reported (about 19 h). Lessons: A causal relationship must be suspected between the fetal anemia and the maternal use of mesalazine. This fetal side-effect should be considered in pregnant women on mesalazine (and its prodrug sulfasalazine).