We evaluated serum neopterin as a biomarker of chronic beryllium disease (CBD), for use in conjunction with the beryllium lymphocyte proliferation test (BeLPT) in workplace screening. Serum neopterin levels were determined by radioimmunoassay, and we compared levels in three groups: CBD (n 5 86), beryllium sensitized (BeS) (n 5 22), and normal (Nor) (n 5 20). Those in the diseased group underwent pulmonary function tests, bronchoalveolar lavage (BAL), and maximal exercise testing. We correlated neopterin levels with results of these clinical parameters of disease severity. To evaluate the optimum sensitivity, specificity, and neopterin cut-off value, receiver operator characteristic (ROC) curves were generated. The median serum neopterin level in CBD was significantly higher than in BeS or in Nor [median 1.45, 25 th , 75 th percentiles (1.00, 2.7) ng/ml, 0.82 (0.67, 1.16) ng/ml, and 0.92 (0.86, 1.16) ng/ml, respectively] (P , 0.05). In CBD, we observed statistically significant associations between neopterin and measures of gas exchange and BAL cellularity. Using a neopterin value of 1.27 ng/ml, test specificity is 88%. In those workers with an abnormal BeLPT, serum neopterin has high positive predictive value (92%), and can identify disease, helping to distinguish it from BeS without the risks of biopsy.