The current study aimed to identify risk factors for atrial fibrillation (AF) detected during routine medical checkups of male workers in Japan. A nested case-control study was conducted using retrospective data from January 1998 to December 2006 collected at a hospital in Ishikawa Prefecture. Cases were those first diagnosed with AF with no record of AF events during the preceding 3 years. For each case, 2 controls were matched for age and time of medical checkup, randomly selected from among those who had not been diagnosed with AF during the same 3 years. Logistic regression was used to identify risk factors in the 3 years prior for new-onset AF. Sixty-nine cases and 138 controls were recruited; their average ages were 57.6 (SD 6.7) and 57.4 years (SD 6.7), respectively. In the logistic models, new-onset AF was associated with systolic blood pressure and drinking habits in the 3 years prior.