We determined the occurrence of birth rust fungus (Melampsoridium betulinum) within mature mountain birch (Betula pubescens var. czerepanovii) trees and compared the airborne urediniospore concentrations with overall rust infection levels over five successive years. Fifteen groups of three trees 3–5 m high were chosen from a homogenous mountain birch stand. Each group contained a tree with ‘low’, ‘moderate’ and ‘high’ infection covering the natural range of infection levels. The difference in rust classes remained constant throughout the study; trees in class ‘high’ consistently had always highest infection levels and trees in class ‘low’ always the lowest infection levels. Infection levels remained constant between years in ‘low’ class, while the infection levels varied significantly between years in ‘moderate’ and ‘high’ classes. The incidence of disease caused by M. betulinum varied from year to year and was correlated with the total airborne urediniospore counts. The data indicate that trees differ in resistance to birch rust and that infection levels of susceptible compared to resistant trees are more dependent on environmental conditions.