Large health disparities exist in the U.S. across ethnic and socioeconomic status groups. Using nationally representative data, we tested whether American patients with diet-related chronic diseases had higher diet quality than nonpatients. We also tested whether nutrition knowledge and beliefs (NKB) and food label (FL) use were associated with the observed differences. The 1994-1996 Continuing Survey of Food Intake by Individuals, and the Diet and Health Knowledge Survey were examined for 4356 U.S. adults. Dietary intakes were assessed using 2 nonconsecutive 24-h recalls and diet quality was assessed by using the USDA 2005 Healthy Eating Index (HEI). Patients' mean HEI was higher than that of nonpatients (mean ± SE: 53.6 ± 0.5 vs. 51.8 ± 0.4; P < 0.001). Among patients, blacks were 92% more likely to report low diet quality (HEI < 20th percentile) than whites. The positive association between chronic diseases and HEI was observed only for patients with good NKB [OR = 1.80 (95% CI = 1.34, 2.43)]. The diabetes-HEI association was stronger among FL users [OR = 2.24 (95% CI = 1.08, 4.63)] than non-FL users [OR = 1.33 (95% CI = 0.65, 2.73)]. Hypertensive patients' and nonpatients' diet quality did not significantly differ; linear regression models showed no difference in their HEI (β ± SE: 0.6 ± 0.6; P > 0.05) or sodium intake (-18.6 ± 91.4 g/d; P > 0.05) between them. In conclusion, U.S. adults with diet-related chronic diseases reported somewhat higher diet quality than nonpatients, especially among those patients with good NKB and use of FL. Efforts are needed to promote healthy eating among Americans with diet-related chronic diseases; nutrition education and promotion of FL use may help.