Case-referent studies nested within occupational cohort studies are efficient designs for estimating relative risks associated with exposures and for obtaining data on nonoccupational confounding factors. Frequently, however, only incomplete data on confounders can be obtained from sources such as next-of-kin or co-workers. A method for assessing the potential extent of confounding in cohort studies described previously by Axelson [Scand J Work Environ Health 4 (1978) 98-102] can be adapted for use in nested case-referent studies. An example is shown to illustrate that the adapted Axelson method is preferable to the usual confounder adjustment techniques when data on the confounder are incomplete.