Cook fettuccine in a pasta pot of boiling salted water (2 tablespoons salt for 6 quarts water) until al dente. Reserve ½ cup cooking water, then drain pasta.
Meanwhile, bring cream and butter to a simmer in a 12-inch heavy skillet over medium-low heat, adding ¼ teaspoon salt and ½ teaspoon pepper.
Add fettuccine, ¼ cup reserved water, and cheese to sauce and toss. Add more cooking water if necessary.
