The Consumer Confidence Index, which had previously decreased in April, fell again in May, The Conference Board announced Tuesday. But U.S. consumers remain optimistic on the whole.

The index now stands at 117.9, down from its April reading, the group said. Economists were expecting the index to drop slightly to 119.8 for the month of May, according to Thomson Reuters consensus estimates.

The Consumer Confidence Index last fell to 120.3 in April after hitting 125.6 in March, its highest level since December 2000.

"Consumer confidence decreased slightly in May, following a moderate decline in April," Lynn Franco, director of economic indicators at The Conference Board, said in a statement.

"However, consumers' assessment of present-day conditions held steady, suggesting little change in overall economic conditions. Looking ahead, consumers were somewhat less upbeat than in April, but overall remain optimistic that the economy will continue expanding into the summer months."

Economists closely monitor the mood of consumers because their spending makes up about 70 percent of U.S. economic activity.