Insulin-dependent diabetic pregnant women are at risk for magnesium deficiency, predominantly because of increased urinary magnesium losses. They also have a high incidence of spontaneous abortion, possibly related to major lethal malformations. We tested the hypothesis that adverse fetal outcome (fetal loss before 20 weeks' gestation and/or congenital major malformations) is related to magnesium status (as assessed by determining serum magnesium levels) in insulin-dependent diabetic pregnant women, even after sonographic documentation of fetal viability. Eighty-four insulin-dependent diabetic women (class B to RT) with 96 pregnancies were recruited prospectively in a program project. Serum magnesium and blood glycohemoglobin were measured at about nine weeks' gestation. Blood glycohemoglobin was higher (P = .039) and serum magnesium concentration lower (P = .05) in the 21 pregnancies that ended in adverse fetal outcome, compared with the other (75) successful pregnancies. When compared with the "successful pregnancy" group, blood glycohemoglobin was higher (P = .012) and serum magnesium lower (P = .037) in the subgroup of nine pregnancies with fetal cardiac activity present by ten weeks and ending in adverse fetal outcome, compared with the 64 equivalent pregnancies in the "successful" group. We speculate that decreased magnesium status may contribute to the high spontaneous abortion and malformation rate in insulin-dependent diabetic pregnant women.