Nonexperimental studies using computerized databases often give rise to missing or partially available information on confounders. A frequent situation occurs when data on exposure are available for all subjects of a case-control study, but data on confounders are available only for the cases but not for the controls. In that situation, the fact of confounding can be verified by assessing the association between exposure and a confounder in the cases, but the data are insufficient to produce an adjusted estimate of the relative risk if confounding is found to be present. We propose simple conditions under which an adjusted estimate of the relative risk can be obtained when data on a confounder are available only for the cases, and we derive formulae for the estimator and its confidence limits. The method requires an external estimate of the confounder prevalence or, additionally, of the confounder-exposure odds ratio. We illustrate the technique with data from a nested case-control study of the risk of acute cardiac death associated with the use of bronchodilator drugs within a cohort of 12,301 asthmatics, with smoking as the confounder of interest.