Extra genetic material that is euchromatic is generally regarded to be associated with phenotypic abnormalities. However, recent studies suggest that this is not always the case. Chromosome analysis was performed on amniotic fluid cells from a 37-year-old phenotypically normal patient referred for advanced maternal age. All the cells analysed showed a karyotype of 46,XY,1p+. The 1p+ chromosome had extra genetic material of uncertain origin in chromosome band region 1p21-->31. Chromosome analysis on the father revealed a normal 46,XY male karyotype. The mother's karyotype showed the same 1p+ chromosome. C and Q banding, as well as silver staining studies, in both the mother and the fetus support the interpretation that the extra chromosomal material was euchromatic in nature. This 1p+ chromosome may be characterized as a euchromatic heteromorphism. Euchromatic heteromorphisms not associated with phenotypic abnormalities have been reported for chromosomes 9 and 16. To the best of our knowledge, this is the first report involving this type of cytogenetic anomaly on chromosome number 1 in a phenotypically normal mother and infant.