A liveborn girl with 46,XX/47,XX+4 mosaicism is reported for the first time. The diagnosis of true mosaicism was established initially in the assay of cultured amniotic fluid cells, although fetal blood obtained by percutaneous umbilical blood sampling showed a 46,XX chromosome constitution. The liveborn infant had manifestations previously reported in dup(4p) and dup(4q) syndromes. Cells in cord and arterial blood samples also were 46,XX, but cultures of placenta and bilateral forearm skin biopsies showed 46,XX/47,XX,+4 mosaicism. This case illustrates the disadvantage of chromosome analysis from blood alone when tissue-specific mosaicism is present.