In addition to the therapeutic trial a serologic follow-up substudy was conducted on 25 percent of patients enrolled in the therapeutic trial. The subset of patients were randomized to obtain blood samples at three and six months, one and two years, and at the end of the study for serologies. The purpose of the substudy was to determine the effect of azithromycin on serologic titers to C. pneumoniae and to evaluate whether the occurrence of coronary heart disease events is associated with a change in antibody titer.