At least 3,700 children in Guinea, Liberia and Sierra Leone who have lost one or both parents to Ebola this year face being shunned, the UN has said.
Carers were urgently needed for these orphans, Unicef said.
A basic human reaction like comforting a sick child has been turned "into a potential death sentence", it added.
The World Health Organization (WHO) says more than 3,000 people have died of Ebola in West Africa - the world's most deadly outbreak of the virus.
The figure on the number of Ebola orphans follows a two-week assessment mission by the UN children's agency to the three countries worst-affected by the outbreak. An earlier version of this story said that 4,900 children had lost parents but the correct figure is 3,700.
It found that children as young as three or four years old were being orphaned by the disease.
Children were discovered alone in the hospitals where their parents had died, or back in their communities where, if they were lucky, they were being fed by neighbours - but all other contact with them was being avoided.
"Thousands of children are living through the deaths of their mother, father or family members from Ebola," Unicef's Manuel Fontaine said in statement about his two-week visit to the region.