We measured 24-hour ambulatory blood pressure and 16-hour noise exposure continuously for 20 automobile workers, and used linear mixed-effects regression models to estimate transient and sustained effects of noise exposure on blood pressure. The occupational noise levels of the high-exposure workers with 85 +/- 8 dBA were significantly higher than those of the low-exposure workers with 59 +/- 4 dBA (P < 0.05). We found a significant difference of 16 +/- 6 mm Hg in sleep-time systolic blood pressure (SBP) existed between 2 exposure groups, and a marginal increase of 1 mm Hg SBP per 1-dBA increase in occupational noise exposure at a 60-minute lag time during work (P = 0.07). Occupational noise exposure had both transient and sustained effects on workers' SBP.