A cohort mortality study was conducted among 3,779 employees at a petroleum exploration and extraction research facility to evaluate workplace exposures and brain tumor risk. Deaths were identified by searches against the National Death Index, Social Security Administration, and California state mortality files. Work histories were classified by job titles, laboratory activity, and company division. Eleven brain tumor deaths were observed among the cohort (standardized mortality ratio [SMR] 1.8; 95% confidence interval = 0.9-3.2). SMR analyses for scientists, employment in laboratory work, and in the research division were not associated with an increased brain tumor SMR, whereas an increased SMR was observed for administrative and nonresearch employees. Although conclusions are limited by the small study population and lack of specific exposure data, these findings were not consistent with an occupational explanation for the observed brain tumor cluster.