Serum carcinoembryonic antigen (CEA) levels were determined by the Hansen-Z-gel technique in 276 healthy volunteers, of whom 154 were smokers and 122 nonsmokers. The mean CEA level was significantly higher in smokers (2.7 ng/ml) than in nonsmokers (1.9 ng/ml) (P less than .001), and a significantly higher percentage of smokers had elevated CEA levels (P less than .05). In both groups, CEA levels were directly related to age. Seventy-six of the 154 smokers who entered the study ceased smoking. Their CEA levels were determined at one, three, and six months after cessation of smoking. Within three months after cessation, elevated CEA levels declined to within the range of nonsmokers and did not appear to be influenced by previous smoking habits. Both age and smoking history must be considered for accurate evaluation of CEA levels. A reappraisal of the diseases associated with elevated CEA levels that considers the influence of age and smoking may invalidate some of the correlations previously reported.