Drinking water and dietary sources of nitrate and nitrite can react in vivo with amines and amides to form N-nitroso compounds (NOC), potent animal carcinogens. Nitrate is a widespread contaminant of drinking water supplies especially in agricultural areas. We conducted a population-based case-control study of renal cell carcinoma in 1986-1989 in Iowa, a state with elevated levels in many public water supplies. We collected a lifetime water source history, but due to limited monitoring data, most analyses focused on the subpopulation, who used Iowa public supplies with nitrate measurements (actual or imputed data) for > or = 70% of their person-years since 1960 (201 cases, 1,244 controls). We computed the average nitrate level and years using a public supply with nitrate levels >5 and >10 mg/l. Dietary nitrate and nitrite were estimated from a 55-item food frequency questionnaire. There was no association of renal cell carcinoma with the average nitrate level and years using public supplies >5 and >10 mg/l nitrate-nitrogen (10+ years >5 mg/l odds ratio (OR) = 1.03, 95% confidence interval (CI) 0.66, 1.60). However, higher nitrate exposure was associated with an increased risk among subgroups with above the median red meat intake (10+ years >5 mg/l OR = 1.91, 95% CI 1.04-3.51) or below the median vitamin C intake (10+ years >5 mg/l OR = 1.90, 95% CI 1.01, 3.56), dietary factors that increase the endogenous formation of NOC. Exclusion of long-term Des Moines residents, a large proportion of the high exposure categories, attenuated the association. These findings deserve additional study in populations with high water nitrate intake and information on dietary intakes.