Three new cases of true mosaic trisomy 17 (MT17) were diagnosed in amniotic fluid cells. Postnatal chromosome analysis from lymphocytes did not confirm the trisomic cell line, and follow-up studies showed normal psycho-motor development of the children, in one case up to the age of 4 1/2 years. We suggest that there are similarities between MT17 and MT20, in which the majority of pregnancies result in deliveries of healthy babies.