Rapid advances in genetic epidemiology and the setting up of large-scale cohort studies have shifted the focus from severe, but rare, single gene disorders to less severe, but common, multifactorial disorders. This will lead to the discovery of genetic risk factors for common diseases of major importance in insurance underwriting. If genetic information continues to be treated as private, adverse selection becomes possible, but it should occur only if the individuals at lowest risk obtain lower expected utility by purchasing insurance at the average price than by not insuring. We explore where this boundary may lie, using a simple 2 × 2 gene-environment interaction model of epidemiological risk, in a simplified 2-state insurance model and in a more realistic model of heart-attack risk and critical illness insurance. Adverse selection does not appear unless purchasers are not very risk-averse, and insure a small proportion of their wealth; or unless the elevated risks implied by genetic information are implausibly high. In many cases adverse selection is impossible if the low-risk stratum of the population is large enough. These observations are strongly accentuated in the critical illness model by the presence of risks other than heart attack, and the constraint that differential heart-attack risks must agree with the overall population risk. We find no convincing evidence that adverse selection is a serious insurance risk, even if information about multifactorial genetic disorders remains private.