The Dow Jones industrial average was down 107.6 points, or 0.83%, shortly after the opening bell.
The National Federation of Independent Business reported that its Small Business Optimism index in April rose 2 points to 94.5, following a decline in March. The index returned to where it was in February 2011.
The results were based on a survey of 1,817 businesses reporting higher sales and plans to increase hiring.
Financial markets have been unsettled by the continued turmoil in Europe. Elections in France and Greece over the weekend called into question fiscal austerity plans aimed at fixing the Eurozone's debt crisis.