Doctors for the Democrat found B-cell lymphoma that will require treatment over the next few months, spokesman Caley Gray said in a news release.
Lautenberg will undergo six to eight chemotherapy treatments and should make a "full and complete recovery," said Dr. James Holland of New York City's Mount Sinai Medical Center.
Lautenberg was taken to the hospital Monday after his office said he fell. The office said Tuesday the senator was treated for a bleeding ulcer, and Gray said Friday that doctors still believe that diagnosis was also accurate.
Lautenberg is expected to return to work at the Senate between treatments, Holland said.
"I wouldn't be too surprised to soon hear how he's once again outpacing younger aides as they walk through U.S. Capitol building," said state Assembly Speaker Sheila Y. Oliver, a Democrat.
After Christie was elected governor in November, some Democratic state lawmakers moved to change the way senate vacancies are filled so the governor would have to name a replacement from the party of the departing senator. They did not adopt the change.
If there is a vacancy, the governor could name a replacement of his choice, call for a special election or leave the seat open.
The liberal Lautenberg became prominent as a founder of the payroll services company Automatic Data Processing long before he entered politics in 1982 with a successful run for the U.S. Senate.
He retired from politics and did not seek re-election in 2000, but returned two years later as a last-minute replacement in a Senate race when Sen. Robert Torricelli dropped out. He was re-elected in 2008, winning easily in a race in which his age never materialized as a major issue.
He is a major supporter of gun control and a big critic of the tobacco industry. He's also active in transportation issues. Lately, he has criticized the Transportation Security Administration over a disruptive security breach at Newark Liberty International Airport.
Lymphoma is an immune system cancer, and the B-cell form is a type of the non-Hodgkin's lymphoma that strikes more than 65,000 people in the U.S. annually. There are multiple subtypes of the B-cell form, with widely varying treatments and prognoses. Lymphomas can strike in lymph tissue anywhere in the body, such as the lymph nodes -- and the stomach contains lymphoid tissue.