Since the incidence of tuberculosis is steadily declining in Finland and infections by environmental mycobacteria may be increasing, the aim of the present study was to evaluate the development of tuberculin reactivity and sensitization to environmental mycobacteria. Healthy Finnish schoolchildren aged 10.4-12.4 yrs (n=201) were tested with tuberculin purified protein derivative RT23, Mycobacterium scrofulaceum RS95 and M. fortuitum RS20 sensitins. The same children had been previously tested with the same antigens and methods at the age of 4-6 yrs in 1989. Rapid waning of tuberculin reactivity and decrease in sensitization to environmental mycobacteria were observed between 4-6 yrs. Both tuberculin and sensitin skin reaction sizes decreased significantly over the 6-yrs period. The mean tuberculin skin reaction size was 3.2 mm in diameter, which was significantly (p