We studied tuberculin reactivity in young Norwegian adults and its possible dependency on age, gender, previous BCG vaccination, smoking habits, occupational exposure, diet as well as years of education as a measure of socio-economic status. Responders of a random sample of men and women aged 20-44 years living in Bergen, Norway were interviewed and tested withthe adrenaline-Pirquettest with Norwegian-produced synthetic mediumtuberculin at the out-patient chest clinic in the city of Bergen in 1992-1993. Nine hundred and three subjects out of 1200 met for the clinical examination (75%). Five hundred and eighty-eight subjects were tuberculin-tested and read, whereof 95% were BCG vaccinated by age 14. Mean tuberculin reactivity was 4.8 mm (SD: 3.0 mm). A positive reaction (> or = 4 mm) was found in 64%, whereof 7% had a strongly positive reaction (>10 mm). A negative reaction (<4 mm) occurred in 36%, whereof 10% had no reaction (0 mm). Only 30% ofthe females and 36% of the males aged 21--25 years were tuberculin positive 7-12 years after BCG vaccination. Linear regression analysis demonstrated tuberculin reactivity to increase with increasing age, male gender with an increasing sex effect by age, and current smoking. Occupational dust or gas exposure, a diet rich in vitamin C or years of education did not influence tuberculin reactivity significantly.