Using several examples of genetic marker and disease associations in the workplace, the authors have applied formulas to estimate the sensitivity, specificity, and positive predictive value (PPV) of screening for these markers. Sensitivity, specificity, and PPV are affected independently by characteristics of the population being screened, ie, the genetic marker frequency, the disease frequency, and the magnitude of the relative risk (R). For a given disease frequency, when the genetic marker is less frequent than the disease, PPV increases with relative risk, although sensitivity remains low. When the genetic marker is more frequent than the disease, PPV remains low while sensitivity increases with R. When marker and disease frequencies are equal, PPV and sensitivity are equal and increase with R. However, when the disease frequency is very low, R must approach 100 before PPV or sensitivity approaches 50%. These relationships may be used effectively in the decision whether to implement a screening program in the workplace.