Independent experts cautioned that the improvement was small, and needs to be verified in another study.
"I hate to get too enthusiastic ... there's a flicker of a signal" of benefit, but less than what some other once-promising treatments showed, said Dr. Sam Gandy, head of Alzheimer's disease research at Mount Sinai School of Medicine in New York.
Dr. Ronald Petersen, director of Alzheimer's research at the Mayo Clinic, called the drug's effect "subtle" and said it may mean just that "somebody remembers one extra word out of a 15-word list" without any real improvement in how well they live.
The drug, if ever approved, is likely to be expensive, and that means "we need more evidence" of its benefit to justify its use, Petersen said.
Encouragingly, solanezumab had few side effects. About 1 percent of people on the drug had some chest pain from reduced blood flow to the heart. There were few cases of worrisome brain swelling and small bleeding in the brain, an effect that caused concern with another experimental Alzheimer's drug, bapineuzumab by Pfizer Inc. and Johnson & Johnson's Janssen Alzheimer Immunotherapy unit.
That drug failed to help patients in two late-stage studies announced last month, but did show signs of hitting its target and clearing deposits from the brain.
The Lilly drug seems to be safer, and that is an advantage, said Dr. Norman Relkin, head of a memory disorders program at New York-Presbyterian Hospital/Weill Cornell Medical Center.
"There is some cause for encouragement here. It's not the magnitude we'd like to see" but certainly warrants further studies on the drug, he said.
Relkin heads testing of the third drug in late-stage development -- Gammagard, by Baxter International Inc. Results are expected early next year.
Meanwhile, Dr. Eric Siemers, senior medical director for Lilly, said the company will discuss its results and next steps with the Food and Drug Administration.
Lilly shares closed up $2.55, or 5.3 percent, to $50.78. During the day they peaked at $50.94, their highest price since April 2008. (This is also their highest closing price since that month.) The shares are up 19.8 percent since August 24.