The proliferation of information on the Internet poses a significant challenge on humans' limited attentional resources. To attract online users' attention, various kinds of animation are widely used on websites. Despite the ubiquitous use of animation, there is an inadequate understanding of its effect on attention. Focusing on flash animation, this study examines its effects on online users' performance and perceptions in both taskrelevant and task-irrelevant information search contexts by drawing on the visual search literature and two theories from cognitive psychology. In the task-relevant context, flash is applied on the search target; while in the task-irrelevant context, flash is applied on a nontarget item. The results of this study confirm that flash does attract users' attention and facilitates quicker location of the flashed target item in tightly packed screen displays. However, there is no evidence that attracting attention increases recall of the flashed item, as is generally presumed in practice, and may even decrease the overall recall. One explanation is that when users have to use their limited attentional resources on suppressing the distraction of flash, they will have less mental resources to process information. Moreover, the results suggest that processing information about an item depends not only on the attention it attracts per se, but also on the attention that other items on the same screen attract. While flashing an item may not increase the recall of that item, it can reduce the recall of other items (especially the nontarget items) on the screen. Finally, flash has negative effects on users' focused attention and attitude towards using the website. These results have implications for website interface design, online product promotion, online advertising, and multimedia training systems, among others.