We report on cytogenetic studies of a malformed fetus, whose clinical symptoms indicated the diagnosis of triploidy. This was confirmed by chromosome analysis of peripheral lymphocytes of cord blood. Using the C-banding method it was possible to identify the origin of the extra haploid set: marker chromosomes indicate, that nonreduction of the first meiotic division in the father's spermiogenesis most probably leads to triploidy. However, in our case fertilization of the zygote by two sperms cannot be excluded.