We examine a large class of prior distributions to model the dose–response relationship in cancer phase I clinical trials. We parameterize the dose–toxicity model in terms of the maximum tolerated dose (MTD) γ and the probability of dose limiting toxicity (DLT) at the initial dose ρ0. The MTD is estimated using the EWOC (escalation with overdose control) method of Babb et al. We show through simulations that a candidate joint prior for (ρ0,γ) with negative a priori correlation structure results in a safer trial than the one that assumes independent priors for these two parameters while keeping the efficiency of the estimate of the MTD essentially unchanged. Copyright © 2005 John Wiley & Sons, Ltd.