Background: The association between serum digoxin concentration (SDC) and outcomes in women with heart failure (HF) has not been well studied. Aims: To test the hypothesis that the effect of digoxin on outcomes in women with HF is bi-directional and dependent on SDC, as in men, and is modified by ejection fraction (EF). Methods: We studied 1366 female participants of the Digitalis Investigation Group trial in whom data on SDC (ng/ml) were available. We calculated adjusted odds ratios (AOR) and Bonferroni-adjusted 97.5% confidence intervals (CI) for various outcomes at a median follow up of 41 months, in all women and stratified by EF 35%. Results: Compared with placebo (26.9%), 40.3% with SDC ≥ 1.2 (AOR = 1.80; CI = 1.14-2.86; p = 0.004) and 26.6% with SDC 0.5-1.1 (AOR = 1.05; CI = 0.73-1.51; p = 0.762) died. Respective rates for HF-hospitalizations were: placebo (32.8%), SDC ≥ 1.2 (38.0%) and SDC 0.5-1.1 (25.5%). For women with EF < 35% (N = 677), SDC 0.5-1.1 lowered odds for HF-hospitalizations (AOR = 0.63; CI = 0.39-1.00; p = 0.026) without increasing odds for death (AOR = 0.77; CI = 0.47-1.26; p = 0.233). In women with EF ≥ 35% (N = 689), SDC 0.5-1.1 had a borderline association with death (AOR = 1.58; CI = 0.92-2.72; p = 0.058) but not with HF-hospitalization (AOR = 0.95; CI = 0.54-1.66; p = 0.826). Conclusions: As in men, in women with HF, digoxin has a bi-directional effect based on SDC, and the beneficial effects were significant only among women with EF < 35%. © 2005 European Society of Cardiology.