OBJECTIVE Clubbing can be a paraneoplastic manifestation of bronchogenic carcinoma. We assessed a new digital index of clubbing and used it to determine the prevalence of clubbing for different cell types of lung cancer. METHODS Clubbing was assessed by measurement of the thickness of both the base of the nailbed (distal phalangeal depth--DPD) and the distal interphalangeal depth (IPD) of the index finger in a control group compared to patient groups with either chronic obstructive lung disease, or lung cancer. RESULTS Of the 55 normal subjects, no patient had a DPD/IPD ratio of more than 1.05 on either hand, while 11% of the patients with COPD had a ratio of more than one. For the cancer patients, 33% had a ratio greater than one, with 30 of 109 (37%) having a ratio > 1.05 (chi(2) = 17.6, p < 0.0001). There was no difference in the prevalence of clubbing between the 33 squamous cell patients, the 43 adenocarcinoma patients, and the 33 small cell lung carcinoma patients included. CONCLUSIONS Measurement of the interphalangeal and distal phalangeal distance demonstrated that one-third of patients with lung cancer had evidence of clubbing. The type of bronchogenic carcinoma did not appear to affect the proportion of patients with clubbing.