Squamous cell carcinoma antigen (SCC Ag), a recently developed tumour marker, has shown some promise, particularly in squamous cell carcinoma (SCC) of the uterine cervix. To evaluate its usefulness in SCC of the lung, serum SCC Ag levels were measured in 52 patients with a variety of respiratory diseases (both benign and malignant), 17 patients with renal failure and 56 healthy controls. SCC Ag was elevated in 11 of 25 patients with SCC lung and one patient with a mixed adeno-squamous tumour, but in no patients with other malignancies or benign respiratory disease. SCC Ag levels appeared unrelated to either tumour stage or the degree of differentiation. False-positive results were noted in patients with renal failure, with six of eight patients with a plasma creatinine greater than 0.7 mmol l-1 having an elevated SCC Ag level. Because SCC Ag was highly specific in the presence of normal renal function, an elevated level would support early investigation of a patient with suspected pulmonary malignancy. The potential of SCC Ag in typing large cell lung carcinomas (where light microscopic features of SCC are uncertain) warrants further investigation.