"Overall, we are certain Berkshire's performance in the future will fall far short of what it has been in the past," Buffett warned in his annual letter to shareholders, before concluding that he's hopeful "we can deliver results that are modestly above average. That's what we're being paid for."
Berkshire Hathaway on Saturday reported 2003 earnings of $8.15bn, up from $4.29bn in the year-ago period. Revenue rose to $63.9bn, from $42.2bn.
Rising insurance premiums boosted the bottom line of Berkshire, which counts consumer insurer Geico and industry reinsurer General Re among its holdings.
"In recent years ... we've found it hard to find significantly undervalued stocks," Buffett said, but he added that the shortage of cheap stocks doesn't bother him, if he can find the right kind of stocks to buy.
Buffett's annual letter is closely watched because he often flags issues ahead of the herd. Last year, for example, he urged corporate boards to get tough with their CEOs.
His noted Berkshire Hathaway's poor fiscal performance in the previous year, weighted down by reinsurance results tied to the September 11 terror attacks.
Buffett said his future buying will hinge on whether a company has "favourable and enduring economic characteristics; are run by talented and honest managers and are available at a sensible price."