The weak U.S. dollar may help American multinational companies meet profit expectations in the quarters ahead, but the first sign of a reversal could eat away profits at a time when every cent counts.

Companies based outside the United States, of course, face the opposite situation: praying for the greenback to strengthen after more than two uninterrupted years of declining.

This week, Finland's Nokia (NOK1V.HE: Quote, Profile, Research) disappointed investors who sent shares of the world's largest mobile phone maker down 13 percent on Wednesday after the company said its market share in Europe would shrink because of the falling dollar.

(Article continues below)

The flip side was Coca-Cola Co (KO.N: Quote, Profile, Research). The company received an 11 percent, or $206 million, boost to operating income just from currency movements during the first three months of the year.

A strong showing from Caterpillar Inc (CAT.N: Quote, Profile, Research), IBM (IBM.N: Quote, Profile, Research) and other companies that generate a large part of their profits overseas have helped investors get over persistent fears about the impact of a devastating credit crisis on business and consumer spending.

But after the euro <EUR=> climbed 8 percent in the first quarter to record highs against the dollar, the risk has increased that the U.S. currency could bottom out and possibly even strengthen later this year -- turning the tables on companies in the United States and Europe.