Insomnia is one of the most common complaints at worksites, as well as in the general population. This study aims to assess the effect of insomnia on the development of hypertension in Japanese male workers. Using the annual health examination database of a Japanese telecommunication company, eligible middle-aged male participants in the 1994 health examination were followed up until 1998 or the development of hypertension (either initiation of antihypertensive therapy or a systolic blood pressure > or = 140 mmHg and/or a diastolic blood pressure > or = 90 mmHg). The effect of difficulty initiating sleep (DIS) was assessed with a DIS dataset (n=4,794), which included non-DIS (n=4,602) and persistent-DIS (n=192) subjects. That of difficulty maintaining sleep (DMS) was assessed with a DMS dataset (n=4,443), which included non-DMS (n=4,157) and persistent-DMS (n=286) subjects. The incidence of hypertension among persistent-DIS (40.1%; 130.7 per 1,000 person-yr) was significantly higher than that among non-DIS (30.6%; 89.9 per 1,000 person-yr). The incidence of hypertension among persistent-DMS (42.3%; 136.7 per 1,000 person-yr) was significantly higher than that among non-DMS (30.7%; 90.8 per 1,000 person-yr). After adjusting for potential confounding factors (i.e. age, body mass index, smoking, alcohol drinking, and job stress), persistent complaints of DIS and DMS were significantly associated with an increased risk of hypertension (OR=1.96; 95%CI: 1.42-2.70 and OR=1.88; 95%CI: 1.45-2.45, respectively). Persistent insomnia may be a useful predictor of hypertension in Japanese male workers.