The objective of this study was to re-evaluate the adult C(T) reference values determined by five different immunoassays and by introducing criteria for selecting control subjects. A prospective multicenter study. Three hundred and seventy-five clinically euthyroid subjects. We used five different C(T) immunoassays. Sera were assayed for the concentration of TSH, gastrin, procalcitonin, urea, calcium, and anti-thyroperoxidase antibodies. Screening for the various potential causes of hypercalcitoninemia led to the exclusion of 23% of the sera. Our reference value analysis dealt with 287 subjects (142 men and 145 women). The proportion of samples in which no C(T) was detected varied from 56% (for assay D) to 88% (for assay C). We observed significant correlations (whose magnitude depended on the assay used) between C(T) levels and age or body mass index (BMI) (primarily in men). The distribution of C(T) levels showed that 4.7, 9.8, 2.5, 6.5, and 8.0% of the values were over 10 pg/ml respectively. These values corresponded essentially to samples from 11 male subjects (median age: 55 years), most of whom were smokers. The highest C(T) values were around twice as high in men than women, and were higher in smokers than non-smokers. Conclusion In clinical practice (and after having excluded the usual causes of raised C(T) levels), the interpretation of C(T) assay results must take into account i) the method used; ii) the patient's gender, age, and weight; and iii) the potential influence of cigarette smoking.