Monozygous twinning has been associated with brain lesions such as hol
oprosencephaly, hydranencephaly, multicystic encephalomalacia, and por
encephaly. Polymicrogyria is rare but has been reported in autopsied c
ases. We present a monozygotic twin with complicating twin-twin transf
usion syndrome, the donor exhibiting normal psychomotor development an
d the recipient psychomotor delay associated with the brain anomaly, p
olymicrogyria. The brain anomaly might be due to blood pressure instab
ility or a circulatory disturbance resulting from increased coagulabil
ity due to polycythemia.