BEIJING — The mother of China’s prime minister was a schoolteacher in northern China. His father was ordered to tend pigs in one of Mao’s political campaigns. And during childhood, “my family was extremely poor,” the prime minister, Wen Jiabao, said in a speech last year.

But now 90, the prime minister’s mother, Yang Zhiyun, not only left poverty behind, she became outright rich, at least on paper, according to corporate and regulatory records. Just one investment in her name, in a large Chinese financial services company, had a value of $120 million five years ago, the records show.

The details of how Ms. Yang, a widow, accumulated such wealth are not known, or even if she was aware of the holdings in her name. But it happened after her son was elevated to China’s ruling elite, first in 1998 as vice prime minister and then five years later as prime minister.