We describe a family with a distinctive malformation of the hand consisting of the fusion of the 4th and the 5th metacarpal bones. Usually this anomaly is clinically recognizable by an ulnar deviation of the 5th finger; moreover, the 5th metacarpal is usually hypoplastic and the 5th ray is con-sequently short. There is, however, great variability in expression, so the degree of fusion may range from minimal to complete and also the external aspect of the hand may vary. This anomaly can be either isolated or part of a syndrome. For the isolated form, two possible hereditary mechanisms have been proposed: autosomal dominant and X-linked recessive. Our family is consistent with the latter, with only affected males and no instances of male-to-male transmission. Of note, there are very few X-linked recessive disorders that affect the hand in a such a specific way. ß 2003 Wiley-Liss, Inc.