Macrophage inhibitory cytokine 1 (MIC 1) is thought to have immunomodulatory actions favouring fetal viability. We measured serum concentrations of MIC 1 in asymptomatic women at 6-13 weeks' gestation who subsequently miscarried or who had already miscarried. MIC 1 concentrations in the miscarriage cohort (n=100), were a third of those who had ongoing pregnancies (n=197). Multiples of the median for miscarriage was 0.32 (95% CI 0.23-0.32) versus 1.00 (0.93-1.06) for ongoing pregnancies; p<0.0001. Concentrations were just as low 3 weeks before diagnosis as on the day of diagnosis. That MIC 1 serum concentrations seem to be low weeks before miscarriage suggests possible predictive and causative roles, as well as therapeutic potential.