We conducted a case-control study in an industrial city in Taiwan to determine whether parents of newly diagnosed patients who were younger than 30 years old with leukemia or brain tumors or the patients themselves were more likely to have been employed in certain occupations or industries. Job histories were collected for parents (and for subjects if they worked) on 103 newly diagnosed cases of leukemia, 74 newly diagnosed cases of brain tumors, and 417 controls matched for age and sex. All jobs since the age of 16 that the subjects held for more than 6 months, a total of approximately 4,000 jobs, were coded for occupation and industry according the standard four-digit system used in Taiwan. Matched-pair analyses were performed comparing cases and controls among all jobs held by subjects and both parents using four-digit occupation and industry codes. Separate analyses also were performed for parental jobs held during the preconception, perinatal, and postnatal periods. Odds ratios (ORs) were adjusted for subject smoking, parental smoking, and exposure to medical radiation. Certain industry and occupation four-digit codes were significantly associated with increased odds ratios of childhood tumors. Including work during any or all periods, leukemias were more common in children of fathers who had worked (1) as wood treaters (adjusted OR 16.03, 95% confidence interval CI = 1.77-145.5), and (2) as building finishers and related trades workers (adjusted OR 4.08, 95% CI = 1.12-14.8), whereas brain tumors were more common in children of mothers who had worked (1) in electronic parts and components manufacturing (adjusted OR 13.78, 95% CI = 1.47-129.0) and 2) as textile and garment workers (adjusted OR 7.25, 95% CI = 1.42-37.0), as well as in subjects who had worked with certain electronic parts and components (adjusted OR 28.67, 95% CI = 2.88-285.6). Leukemias were more common in children of fathers who had worked in the preconception period (1) as wood treaters (adjusted OR 12.17, 95% CI = 1.36-109.2), (2) as building finishers and related trades workers (adjusted OR 4.08, 95% CI = 1.12-14.8), (3) as electronic equipment assemblers (adjusted OR 4.56, 95% CI = I 1.05-19.9), and (4) as certain other assemblers (adjusted OR 10.24, 95% CI = 1.02-102.6). In addition, leukemias were more common in children of fathers who had worked in the perinatal period (1) as wood treaters (adjusted OR 13.08, 95% CI = 1.36-125.5) and (2) as building finishers and related trades workers (adjusted OR 4.51, 95% CI = 1.04-19.6). Brain tumors were more common in children of mothers who had worked in the preconception period (1) in electronic parts and components manufacturing (adjusted OR 11.81, 95% CI = 1.20-116.3), and (2) as textile and garment workers (adjusted OR 7.25, 95% CI = 1.18-31.0).