Paterno, 85, has been in the hospital since Jan. 13 for observation for what his family had called minor complications from cancer treatments. Paterno was diagnosed with cancer in November.
"Over the last few days Joe Paterno has experienced further health complications," spokesman Dan McGinn said in a brief statement. "His doctors have now characterized his status as serious.
"His family will have no comment on the situation and asks that their privacy be respected during this difficult time," he said.
The esteemed football coach was diagnosed with cancer just nine days after his job was swept away in the fallout of former assistant coach Jerry Sandusky's alleged sexual abuse of children.
The Associated Press contributed to this report.