John Shapiro, co-founder and managing director of Chieftain, said his firm sent the letter after failing in other attempts to get management to change.
"Their emphasis has been growth, not return to shareholders," he said. "There's nothing wrong with growth, but if you overpay for it, it dilutes the value to shareholders."
Shapiro said Comcast has spent over $80 billion on acquisitions in the past decade, often paying more than 20 times operating cash flow. And he contended that Comcast spends money on things outside its core cable business, such as regional sports networks, Internet sites and the wireless spectrum.
If Comcast can afford to overpay for acquisitions, Shapiro said, then it should buy back its undervalued shares. But buybacks have so far been "modest relative to their capability," he said.