OBJECTIVE: Digital clubbing is regarded as the oldest clinical sign of medicine. The cause of this unique finger deformity has remained elusive throughout the centuries. For 3 decades our group has studied the etiology of this acropachy. This article reviews the current knowledge on the cause of digital clubbing. METHODS: PubMed database (www.pubmed.gov) was accessed. In clinical queries/clinical study service we entered "clubbing" or "hypertrophic osteoarthropathy," choosing the "etiology" category with a "broad sensitive" search scope. The time span was from January 1975 to August 2006. Additionally, this article narrates the chronology of our research on the pathogenesis of clubbing. RESULTS: The many dreadful internal illnesses associated with digital clubbing have in common enhanced platelet/endothelial cell activation. Emerging evidence suggests that, in hypoxic conditions with extrapulmonary shunting of blood, large megakaryocyte fragments fail to enter the pulmonary circulation. Instead they gain access to the systemic circulation impacting at the most distal sites, there releasing growth factors and thus inducing clubbing. In cases of lung cancer, the purported growth factor could gain direct entrance to the systemic circulation. Vascular endothelial growth factor (VEGF) may play a central role in the development of digital clubbing. It is a platelet-derived factor induced by hypoxia, and it is also abnormally produced by diverse malignant tumors fostering their uncontrolled growth. On the other hand VEGF produces vascular hyperplasia, edema, and fibroblast/osteoblast proliferation. Such are clubbing histologic characteristics. Enhanced VEGF expression has been reported in practically all internal illnesses associated with this type of finger deformity. Recent studies have demonstrated high circulating levels as well as increased local expression of VEGF in different groups of patients with digital clubbing. CONCLUSION: Abnormal expression of VEGF may be the cause of digital clubbing.