Although some cases of spoon nails and clubbing are manifestations of internal disease, a significant number are completely idiopathic and of no consequence to the patient. Many patients have been forced to pay high insurance premiums or have been denied employment because of insignificant nail changes. When a patient has spoon nails or clubbing, a thorough but reasonable search for a primary etiology should be made. It should be recalled that when the complete syndrome of hypertrophic osteoarthropathy is present and joint pain is prominent, the possibility of a bronchogenic carcinoma is great. If a cause of spooning or clubbing is found, it should be treated. However, when the physician becomes satisfied that no associated disorder exists, he should reassure the patient. In the author's experience koilonychia is most commonly due to psoriasis, fungus, or trauma. It is hypothesized that the curvature changes in spoon and clubbed nails are the result of angulation of the nail matrix secondary to connective tissue changes. The structural hypothesis presented here is largely based on observations of congenital and traumatic deformities. These cases have allowed visualization of the angle of the nail matrix. Both spoon nails and clubbing are such definite reaction patterns that there must be some common mechanism for each deformity. Structural rules that should apply to the nail have been used. It is easy to think of multiple internal disorders producing the same connective tissue changes that alter the angle of the nail matrix. It is difficult to think of another mechanism by which internal disease could alter the bonds in hard keratin.