A 22-week-old fetus with trisomy 21 demonstrated intratubular germ cel
l neoplasia. This is the second report of in situ testicular neoplasia
in a fetus with trisomy 21, suggesting that the mechanism responsible
for the possible excess of testicular germ cell tumors that occur in
Down syndrome is operative in early fetal life. Because no examples of
neonatal testicular germ cell turner in trisomy 21 have been reported
, we suggest that this in situ neoplasm may disappear during gestation
, such as the trisomy 21-associated transient myeloproliferative syndr
ome does shortly after birth. This disappearance may be due to a contr
olled regression phenomenon.