There is now strong evidence that megakaryocytes continually emerge from the bone marrow but are trapped by the pulmonary capillary bed and fragment there into platelets. It is suggested that in disorders in which megakaryocytes or megakaryocyte fragments bypass the lung capillary network (eg, right-to-left intracardiac shunts, carcinoma of the bronchus), or in which large platelet clumps form on the left side of the heart or in large arteries (eg, subacute bacterial endocarditis, subclavian aneurysm), or in which there tends to be a chronic platelet excess (eg, chronic inflammatory bowel disease), these large particles may reach the fingertips in axial vascular streams and impact there, releasing platelet-derived growth factor. This material is known to cause increased capillary permeability and connective tissue hypertrophy. It is suggested that this mechanism is the cause of clubbing of the fingers.