Forty-two migrants were taken to hospital in Croatia, including Afghans, after several dozen were found packed in a van travelling through the country, Croatian authorities said.
Emergency doctors said on Sunday that most of the migrants suffered from carbon monoxide poisoning and some were unconscious.
Doctor Maja Grba Buljevic told the Hina news agency that their condition had improved.
Croatia's state TV said a total of 62 migrants, including children, were in the van when it was stopped late on Saturday by traffic police on a road near the town of Novska, by the Bosnian border.
The report said all the migrants were men from Afghanistan and Pakistan smuggled illegally into the country.
It said a police statement was expected later on Sunday.
Thousands of migrants are stranded in neighbouring Serbia looking for ways to reach western Europe.