OBJECTIVE This investigation assessed the validity of estimates of exposure to 1,3-butadiene (BD) developed for a plant included in a study of mortality among synthetic rubber industry workers. The estimates were developed without using historical measurement data and have not been validated previously. METHODS Personal BD measurements came from an exposure-monitoring program initiated in 1977. For each job, we computed the year-specific difference between the BD estimate and the mean of BD measurements. We also computed rank correlation coefficients and calculated the mean, across all measurements, of the difference between the estimate and the measurement. RESULTS The mean BD concentration was 5.2 ppm for 4978 measurements and 4.7 ppm for the corresponding estimates. The mean difference between estimates and measurements was -0.50 ppm (standard deviation, 26.5 ppm) overall and ranged from -227.9 to +27.0 ppm among all 306 job/year combinations. Estimates were correlated with measurements for all 306 combinations (rank correlation coefficient, r=0.45, p<0.0001), for 82 combinations pertaining to jobs that were well-defined by a specific set of tasks and typically found in styrene-BD rubber (SBR) plants (r=0.81, p<0.0001), for 70 combinations pertaining to jobs that were well-defined but not typical (r=0.29, p=0.01) and for 92 combinations pertaining to poorly-defined jobs typically found in SBR plants (r=0.56, <0.0001). Estimates were not correlated with measurements for poorly defined jobs not typically found in SBR plants (r=0.01, p=0.93). For well-defined typical SBR jobs with measurement means that were over 7.0 ppm, estimates were consistently lower than measurements. CONCLUSIONS Possible reasons for differences between estimates and measurements included faulty assumptions used in developing BD estimates, unstable or nonrepresentive measurements and errors in linking measurement data to the job-exposure matrix. Exposure misclassification may have been more severe for subjects from the validation study plant than for subjects from other plants in the mortality study. BD estimates for typical SBR jobs, which comprise most operations at all but one of the plants in the mortality study, appeared to be useful for ranking workers by cumulative exposure. Uncertainty analyses would enhance the utility of the BD exposure estimates for quantitative risk assessment.