To clarify the effect of shift work on blood pressure in Japanese men, a 14-year historical cohort study was conducted in day workers (n=3963) and alternating shift workers (n=2748) who received annual health checkups between 1991 and 2005 in a Japanese steel company. The end points were a >or=10%, >or=15%, >or=20%, >or=25%, or >or=30% increase in systolic blood pressure or diastolic blood pressure from baseline during the period of observation. The association between shift work and an increase in blood pressure was investigated adjusting for age, body mass index, hemoglobin A1c, total serum cholesterol, creatinine, aspartate aminotransferase, gamma-glutamyl transpeptidase, uric acid, drinking habit, smoking habit, and habitual exercise by multivariate pooled logistic regression analyses. Shift work was significantly associated with the various end points. The odds ratios (and 95% CIs) were as follows: >or=10%, 1.15 (1.07 to 1.23); >or=15%, 1.21 (1.12 to 1.31); >or=20%, 1.15 (1.04 to 1.28); >or=25%, 1.20 (1.06 to 1.37); and >or=30%, 1.23 (1.03 to 1.47) for systolic blood pressure and >or=10%, 1.19 (1.11 to 1.28); >or=15%, 1.22 (1.13 to 1.33); >or=20%, 1.24 (1.13 to 1.37); and >or=25%, 1.16 (1.03 to 1.30) for diastolic blood pressure. Our study in male Japanese workers revealed that alternating shift work was a significant independent risk factor for an increase in blood pressure. Moreover, the effect of shift work on blood pressure was more pronounced than other well-established factors, such as age and body mass index.