Seth Wenig / AP file

Former New York City Mayor Ed Koch was hospitalized Saturday because of fluid build-up in his ankles. Here, he speaks with reporters as he is released from the hospital in New York on Dec. 10, two days before his 88th birthday.

Former New York Mayor Ed Koch has been hospitalized for the third time in the last six months.

Koch, 88, was admitted to the hospital around 10 p.m. Saturday with swollen ankles, according to spokesman George Arzt. Arzt said tests on Sunday showed the Koch had some fluid in his lungs. 

Arzt says Koch "sounded great'' when they spoke Sunday. But he added that Koch didn't look well on Saturday at a lunch with former aides. Koch, who was mayor from 1978 to 1989, dined with friends on Saturday night, including a doctor who recommended he seek medical attention. 

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It is unclear when Koch will be released. 

Koch was hospitalized in December with a respiratory infection and in September with anemia.

Before he was mayor, Koch was a congressman from New York, from 1969 to 1977. A decade after leaving the mayor's office, he spent two years as a television show judge on "The People's Court." 

He has remained active on Twitter, where he occasionally tweets as @mayoredkoch. He has urged people to donate to a relief fund for victims of Superstorm Sandy, announced his support of President Barack Obama on the eve of Election Day but also discouraged him from getting involved in the Syrian civil war.

He also thanked New York Mayor Michael Bloomberg for hosting his birthday party at Gracie Mansion. On that day, Dec. 12, he tweeted: "Citizens, thank you for all your birthday wishes. I am 88 years old today and still lucky to live in the greatest city in the world." 

--Reporting by NBC New York staff and NBC News' Isolde Raftery.