Reasoning that they could combat the disease by reducing the levels of very-long-chain fatty acids, they developed an oil derived from rapeseed and olive oil. When they gave the oil to Lorenzo, it reduced the levels of very-long-chain fatty acids in his blood, by blocking the enzymes that make them.
The devotion and determination of Lorenzo's parents helped prolong his life and inspired the film, which came out in 1992. After its release, scientists were angered by suggestions that the oil was a cure for the disease.

About the film:
http://www.imdb.com/title/tt0104756/