During the last 5-6 years our understanding of Chlamydia pneumoniae has changed radically. C. pneumoniae is no longer considered a dangerous, obligatory pathogen. Rather, it is a common, highly contagious intracellular opportunist, inducing poor immunity and with a tendency to repeated reinfections. At present, a possible role in the formation of atheromatous plaques is being discussed. There is a significantly higher prevalence of antibodies against C. pneumoniae in coronary heart disease patients than in controls. Another unsolved problem is that of therapy, since chronic lung infection resists long-term macrolide antibiotic treatment. Should additional treatment with cortisone be given? Here we clearly need clinical trials before we move in a totally new direction.