Colorectal cancer is rare in patients younger than 20 years of age (incidence, 1 in 10 million). Although carcinoembryonic antigen (CEA) and carbohydrate 19-9 antigen (CA 19-9) have been used widely as markers for the efficacy of therapy or detection of recurrent colorectal carcinomas in adults, no studies evaluating their efficacy in children and adolescents have been performed. Between 1986 and 1989, serial measurements of serum CEA and CA 19-9 levels were obtained from 11 patients (aged 8 to 18 years) treated at the institution of the authors. In contrast to the experience in adults, most (9 of 11) of the tumors were localized in the right or transverse colon. Histologically, 10 of 11 carcinomas were mucinous. Most patients had either Dukes' C (5 of 11) or D (5 of 11) lesions. After surgery, all patients were treated with a combination of 5-fluorouracil with Leucovorin (Lederle Laboratories, Pearl River, NY). With the use of the adult normal standards (CEA less than 3.0 ng/ml and CA 19-9 less than 37 ng/ml)6 for the patients, the specificity and sensitivity of CEA were 77% and 64%, respectively, whereas the specificity of CA 19-9 was 79% and the sensitivity was significantly low (10%). The combined specificity of the two markers was 71%, and the combined sensitivity was 60%. On the basis of these results, the authors believe that CEA and CA 19-9 are not effective markers for detection of recurrent or progressive colorectal carcinomas in children and adolescents.