Serum vitamin C levels were compared in smokers and nonsmokers in relation to dietary and supplemental intake of vitamin C, using data from the Second National Health and Nutrition Examination Survey. Smokers reported a lower mean daily intake (53 mg) of vitamin C than nonsmokers (65 mg) and a higher percentage (41% compared to 31%) consuming less than 70% RDA. Smoking status of respondents was judged by carboxyhemoglobin levels or by questionnaire. With both methods, percent of nonsupplemented smokers with serum vitamin C 0.3 mg/dl or less was two or more times as high as nonsupplemented nonsmokers at similar dietary intake levels. When smokers and nonsmokers with similar dietary vitamin C intake were ranked by serum C level, median and mean serum C for smokers was consistently lower than nonsmokers by approximately 0.2 mg/dl. By using the parallel bioassay methods, it was estimated that smokers would need an additional 59 mg/day dietary vitamin C (95% confidence interval of 52-68 mg/day) based on median values or 65 mg/day (53-79 mg/day) based on mean values to attain serum C levels comparable to nonsmokers.