At least 21 people including eight children died after their boats sank off the Greek islands of Farmakonisi and Kalolimnos early Friday, with dozens of other migrants reported missing, the coastguard said.
They said 48 survivors managed to reach the shore of Farmakonisi in the Aegean Sea close to the Turkish coast after the first boat hit rocks around 2:30am (0030 GMT) as it was making the perilous crossing from Turkey.
The coastguard recovered the bodies of six children and a woman.
Several hours later, a second boat capsized off the tiny nearby island of Kalolimnos. The coastguard recovered 14 bodies - two children, nine women and three men.
The 26 survivors reported that dozens of people had been on the boat. The coastguard was searching for the others, backed by a helicopter from EU border agency Frontex.
People fleeing war and misery in the Middle East and elsewhere - many of them Syrian refugees -- are still arriving from Turkey in flimsy boats in their thousands every day, despite the dangers and the harsh winter weather.
The International Organization for Migration estimates that some 31,000 migrants have reached Greece by sea so far this year, hoping to start new lives in Germany, Sweden and elsewhere in the European Union.
The IOM reported 77 people died trying to cross the Mediterranean between January 1 and January 17.
On Thursday at least 12 migrants, including children, drowned off the Turkish coast as their boat tried to reach Greece. The Turkish coastguard rescued 28 people.