Obtaining valid and reliable quantitative exposure estimates is a significant challenge in community-based case-control studies in part, because industrial hygiene monitoring data are usually not available and detailed information on the job and work environment is usually not systematically obtained or assessed. To improve the quality and credibility of disease risk information obtained from occupational case-control studies, we recommend that standardized exposure assessment methods be used to derive quantitative exposure estimates. We identify sources of variation inherent to the assessment process, including: the quality of the information reported on the job, industry, activities, and materials; the industrial hygienist's familiarity with the reported job/industry; the probability that the job/industry was exposed, which depends on plant preferences for particular substances, on process technology, and on customer specifications; and variability in workplace characteristics. To improve the reliability of estimating job-related exposures both within and between studies, we recommend that the epidemiologic analyses be conducted with and without data rated to be of poor quality; that contact be made with experts when the study industrial hygienist is unfamiliar with the manufacturing process in question; that existing data bases be used to estimate the probability of exposure; that a data base be developed that describes manufacturing processes; and that explicit criteria based on industrial hygiene principles be used to evaluate workplace characteristics. In addition, a procedure is described for deriving quantitative exposure estimates by using a reference scale of frequently monitored jobs with their associated mean exposure levels. Areas of research are identified to improve exposure assessment in community-based case-control studies.