The relationship of age and previous BCG vaccination with tuberculin skin test (TST) reactivity was investigated to assess the interpretation of TST results in the adult population of Turkey, where there is a high prevalence of tuberculosis and a routine BCG vaccination programme. The influences of age and BCG vaccine status on booster reaction were also evaluated. TST was applied (5 tuberculin units of purified protein derivative intradermally) to two healthy adult groups, namely 98 medical students and 187 elderly people in a retirement home. The TST was considered positive if an induration > or = 10 mm in diameter was produced. Subjects (41 elderly people and 39 students) with a reaction < 10 mm in diameter were retested 1 week later. There was no significant difference between the students (59.1%) and elderly subjects (58.8%) with respect to positive TST response. No influence of BCG scars on TST reactivity was observed in either group. The booster effect was seen more commonly in the elderly, but the presence of a BCG scar did not influence the booster effect in either group. It was concluded that a positive TST response and booster reaction in adults in high-prevalence countries may be caused by latent tuberculosis rather than previous vaccination.