Invasive pneumococcal disease (IPD) is 3-5 times more common among Navajo adults than in the general US population. The authors conducted a case-control study to identify risk factors for IPD among Navajo adults. Navajos aged > or =18 years with IPD were identified through prospective, population-based active laboratory surveillance (December 1999-February 2002). Controls matched to cases on age, gender, and neighborhood were selected. Risk factors were identified through structured interviews and medical record reviews. The authors conducted a matched analysis based on 118 cases and 353 controls. Risk factors included in the final multivariable analysis were chronic renal failure (odds ratio (OR) = 2.6, 95% confidence interval (CI): 0.9, 7.7), congestive heart failure (OR = 5.6, 95% CI: 2.2, 14.5), self-reported alcohol use or alcoholism (OR = 2.9, 95% CI: 1.5, 5.4), body mass index (weight (kg)/height (m)(2)) <5th (OR = 3.2, 95% CI: 1.0, 10.6) or >95th (OR = 2.8, 95% CI: 1.0, 8.0) percentile, and unemployment (OR = 2.6, 95% CI: 1.2, 5.5). The population attributable fractions were 10% for chronic renal failure, 18% for congestive heart failure, 30% for self-reported alcohol use or alcoholism, 6% for body mass index, and 20% for unemployment. Several modifiable risk factors for IPD in Navajos were identified. The high prevalence of renal failure, alcoholism, and unemployment among Navajo adults compared with the general US population may explain some of their increased risk of IPD.