Hyperextensibility of the proximal interphalangeal joints was noted in 3 males and 6 females in 3 generations of a Japanese family. The proposita, a 14-year-old girl, had hyperextensible proximal interphalangeal joints of the 2nd, 3rd, 4th, and 5th fingers giving "swan-neck" appearance during the extension. She was asymptomatic and had no other features of skin involvement. Her father, younger sister, grandmother, two aunts, and three cousins on the father's side had hyperextensible proximal interphalangeal joints. No instance of male to male transmission was present. The condition was thus inherited as an autosomal or X-linked dominant trait.