A fetus with tetrasomy for the short arm of chromosome 12 due to a de novo accessory isochromosome i(12p) is described. Involvement of the 12p in this chromosome aberration was suggested by banding analysis and substantiated by detection of a marked increase of LDH-B in the fetal fibroblasts. The syndrome shown by this fetus includes many of the minor anomalies described for live-born patients with partial trisomy 12p, and in addition malformations including brachymelia, anal atresia and double kidneys.