Investors haven't been this bullish on US stocks since January 2015

-Professional investors have the biggest portfolio weighting to U.S. stocks in their portfolios since early 2015, according to the latest Bank of America Merrill Lynch Fund Manager Survey.

-The enthusiasm comes despite a number of headwinds that have kept returns in check.

-A BofA strategist said the positioning reflects "peak profits, policy and returns."

Professional investors have suddenly turned optimistic about U.S. stocks again, with bullish sentiment fed by an especially buoyant earnings season that is offsetting other concerns.

Domestic equities are holding the biggest portfolio weight since January 2015, according to the August Bank of America Merrill Lynch Fund Manager Survey, a look at where 243 pros who manage $735 billion are putting their money.