The tuberculin skin test (TST) was conducted in 1,087 employees of Nagasaki University Hospital. The size of erythema (T1) was 27.3 +/- 17.0 mm, and 8.6% of all participants showed < = 9 mm, 49.9% showed 10-29 mm, 41.5% showed > = 30 mm. Laboratory technologists showed significantly larger reactions than other groups. Doctors and nurses working on the tuberculosis ward showed larger reactions than those working on the non-tuberculosis wards. Reactions tended to be larger with aging, and the younger employees in their 20s showed significantly smaller reactions than those in their 40s. The second TST was carried out in 253 employees whose T1 was below 30 mm. The size of erythema enlarged from 16.1 +/- 6.6 mm (T1) to 26.4 +/- 15.4 mm (T2), with the difference (booster phenomenon) of 10.2 +/- 14.4 mm. T2 and T2-T1 were larger in nurses working on the tuberculosis ward than those on the non-tuberculosis wards. It was suggested that laboratory technologists, nurses and doctors especially working on the tuberculosis ward are at risk of tuberculosis infection, and the two-step TST was an essential means to know the baseline reactivity, and to determine recent tuberculosis infection.