Hu was sentenced to 3 1/2 years in April for "inciting subversion against the state." The group said that he suffers from chronic cirrhosis because of liver damage from hepatitis B.
"Hu Jia was incarcerated for doing nothing more than exercising rights expressly guaranteed by China's constitution," said Sophie Richardson, Asia advocacy director at Human Rights Watch. "If the government won't exonerate Hu, it should at least release him to get proper medical care."
The government should also stop harassing Hu's wife, Zeng Jinyan, and their baby daughter, the group said.
Hu was sentenced under a law that makes open criticism of the Chinese government a crime. His activities included co-signing a letter, "The Real China and the Olympics."
Hu began his political activities as an advocate for Chinese infected with the HIV virus.