AbstractEpidemiological studies of disease can make use of ancillary risk‐factors, acquired from individuals outside the disease study. For example, several disease studies might use the same job‐exposure matrix to quantify risks due to occupational exposure to industrial agents. We construct a graphical model to combine a logistic regression disease model with models for the ancillary data and the risk‐factor distribution in the population. We estimate the graphical model using Gibbs sampling, and in simulations compare it with methods of direct substitution into logistic regression.