Naphthalene is an important contaminant in indoor and outdoor air. Acute overexposure can have toxic effects, resulting in hemolysis. There have been no studies evaluating the impact of environmental exposure on red blood cell indices. We examined 1- and 2-hydroxynaphthalene urinary metabolites (NAP1 and NAP2) in non-Hispanic White, non-Hispanic Black, and Mexican-American adults in the USA and their relationship with hemoglobin (Hb) and hematocrit (HCT). Using the 2003-2004 National Health and Nutrition Examination Survey data, weighted generalized linear regression analyses were used to examine the association between Hb (in grams per deciliter) and HCT (in percent) with NAP1 and NAP2 (per 100,000 ng/L). Beta coefficients ± SE are reported. NAP1 and NAP2 were highest in non-Hispanic Blacks, followed by non-Hispanic Whites, and lowest in Mexican-American adults. There was a positive association between NAP1 and Hb (0.39 ± 0.11, p = 0.0034) and HCT (1.14 ± 0.28, p = 0.0009) after adjusting for age, gender, race, education, and smoking. Stratified analysis by smoking showed similar results with the association being stronger for smokers (Hb 0.63 ± 0.23, p = 0.02; HCT 1.43 ± 0.79, p = 0.09) than nonsmokers (Hb 0.34 ± 0.14, p = 0.03; HCT 1.08 ± 0.42, p = 0.02). The association was also stronger for non-Hispanic blacks (Hb 0.54 ± 0.20, p = 0.02; HCT 1.43 ± 0.55, p = 0.02) than for non-Hispanic whites (Hb 0.37 ± 0.18, p = 0.06; HCT 1.20 ± 0.51, p = 0.03) and was not significant for Mexican-Americans (Hb 0.30 ± 1.7, p = 0.10; HCT 0.99 ± 0.52, p = 0.08). NAP2 was not significantly associated with Hb or HCT. The observed disparity in NAP1 and NAP2 levels by race/ethnicity is consistent with published literature. The origin of these differences in exposure is unclear but may reflect differences in environmental exposure as well as genetic susceptibility. The positive association between NAP1 with HCT and Hb is an unexpected finding. Further research is needed to understand the possible biological mechanisms or other explanations for this association.