A case of urticaria pigmentosa occurred which involved only one of two identical twins. Identical serum protein and blood typings strongly indicate that the twins are monozygotic. Of 21 pedigrees more than one member is affected with urticaria pigmentosa. Examination of these pedigrees has suggested to various investigators that genetic factors are implicated in this disorder. The finding of discordance for this condition in monozygotic twins does not support the conclusion that this is definitely a genetic disorder.