American Telephone & Telegraph Co. reported yesterday that profits last year rose 18 percent to a record $5.27 billion -- the most ever earned in any year by a private corporation.

Outgoing Chairman John D. de-Butts characterized his final year at the top of the communications giant as "in many ways the Bell System's most successful year."

Sales rose 12 1/2 percent to $41 billion, expenses were up 12.7 percent to $26.5 billion and profits were equal to $7.74 a share.In the previous year, AT&T had earned $4.48 billion ($6.8l a share).

The earnings reported yesterday were reduced by $72 million in 1978 and $63.5 million in 1977 (11 cents a share for both periods) to reflect the possibility that a California Public Utilities Commission refund order will be implemented, making AT&T subsidiary Pacific Telephone ineligible for federal tax benefits from accelerated depreciation and investment tax credits.

In a final message to 2.9 million stockholders of AT&T, the most for any publicly traded firm, deButts said:

Long distance call volume rose 12.6 percent in 1978 to 14.6 million messages and the number of telephones in service rose 5 million to 133.4 million. AT&T serves more than three-quarters of U.S. telephones.

AP&T's return on average capital invested of $82.6 billion was 9.7 percent in 1978 compared with 9.1 percent in 1977.

Capital expenditures for new services and equipment was $13.7 billion last year and is expected to total $14.8 billion in 1979, an increase of 8 percent.

DeButts, who steps down Wednesday night as AT&T's chief executive, noted that computer-based operating systems have enabled his firm to handle a volume of business about 70 percent larger than in 1971 -- when he took over -- but with an increase of just 4 percent in employes.