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Background: Two recent changes in implantable cardioverter defibrillator (ICD) practice are the increased use of single coil leads and less frequent defibrillation threshold (DFT) testing at implantation. Single coil leads have higher mean DFTs which is of unclear clinical significance. Such leads were used in only a minority of subjects in SIMPLE, which showed no benefit of DFT testing. Hypothesis: Clinical characteristics can be used to predict patients at increased risk of an inadequate defibrillation safety margin with single coil leads. Methods: This analysis included 174 consecutive patients undergoing de novo left pectoral, single coil ICD implantation. A standardized modified step down testing algorithm at implantation was used to determine the delivered energy DFT. Logistic regression was used to assess predictors of DFT and of a high DFT (> 25 J). Results: The patient population was 71.3 percent male and 35 percent African American (AA). The mean age was 60.8 ± 14.3 years and ejection fraction was 34.3 ± 15.6 percent. Of twelve clinical factors assessed, the univariate predictors of DFT were age (p < 0.001), male sex (p < 0.001) and AA race (p < 0.001). After covariate adjustment, race, age and ejection fraction were found to be associated with a high DFT. The mean DFT (J) among AA was 16.1 ± 8.8 compared with 13.3 ± 6.5 for the rest of the cohort (p < 0.05). Moreover, 13.1% of AAs had a high DFT compared with 1.7% for the rest of the cohort (p < 0.005). Conclusions: Age, race and sex predict DFTs with single coil leads. AA are at increased risk for a high DFT and thus an inadequate defibrillation safety margin (< 10 J) and should be considered for dual coil leads and/or defibrillation testing at implantation.