A study was done of HIV-negative women aged ≥ 50 years in Harris County between January 1998 and December 2000 who had at least one positive Mycobacterium avium complex (MAC) culture from a pulmonary source, as obtained from the records of hospitals and clinics in the county. Only women with incident cultures positive for MAC for the 3-year collection period were included. Cultures were identified as positive for MAC by growth on solid media or broth methods (BACTEC; Becton Dickinson; Sparks, MD), and results confirmed using a DNA probe technique. A laboratory and population-based surveillance program for Mycobacterium tuberculosis and MAC had been in place since 1997 in Harris County through cooperative efforts between the Baylor College of Medicine, the University of Texas Health Science Center, about 40 Houston-area hospitals, and the Centers for Disease Control and Prevention in Atlanta, GA. This was done as part of the Houston Tuberculosis Initiative, which tracks tuberculosis in the Houston metropolitan area. A comparison population was selected from tuberculosis suspects found throughout Harris County. A hybrid study design using both cross-sectional and case-control methodologies was used. There were 136 subjects with positive MAC culture findings identified and included in the study, and 136 control subjects who had been tuberculosis suspects. The self-declared race of the MAC culture-positive women was “white” in 125, “black” or “African American” in 7, and “Hispanic” in 11. After various risk factors were controlled for, subjects with positive MAC cultures were more likely to be white (odds ratio, 4.6; 95% confidence interval, 2.3 to 9.2). In addition, they were more likely to have bronchiectasis, scoliosis, and pleural disease and lung cavitation on radiography than the control subjects. Cavitation was especially interesting because that traditionally had not been part of the Lady Windermere syndrome. Whether this represented a true genetic difference in susceptibility, and/or differing environmental exposures, or some combination remains unknown at the present time, although it would seem likely that genetics might have had a considerable role.