Rida Hilal, 46, known for his pro-Western editorials, went AWOL on Monday.
"We are very concerned, we urge the security services to solve the mystery" of vanishing journalist Rida Hilal, union board member Gamal Fahmy told French news agency AFP.
He was last heard of when he ordered a kebab to be delivered to his home in downtown Cairo.
The building watchman who brought the meal said the journalist did not open the door. Police later found the flat empty and the door locked from outside.
"If his disappearance is linked to his opinion, it would be a very dangerous precedent"
Gamal Fahmy, Journalists' Union
Police sources said the security services were investigating the case but were unable to say whether his disappearance was personally, criminally or politically motivated.
Fahmy played down the possibility that Hilal had been kidnapped because of his weekly opinion columns.
"If his disappearance is linked to his opinion, it would be a very dangerous precedent" in Egypt, Fahmy said.
The journalist was one of the few in Egypt to have displayed little or no opposition to the US-led war in Iraq.