In this paper, we seek to account for modest and inconsistent empirical support for a positive relationship between team autonomy and team performance by proposing that team task uncertainty impacts on team performance and moderates the impact of increased autonomy. Task uncertainty is defined in terms of a team's lack of prior knowledge about which operational problems will arise when, and the best way of dealing with them. Results from a longitudinal field study of 17 wastewater treatment teams showed that higher levels of task uncertainty were initially associated with reduced performance, assessed in terms of the quality of treated effluent produced by the teams. An intervention designed to enhance team autonomy led to general improvements in team performance, though moderated by team task uncertainty. Under conditions of enhanced team autonomy, a positive relationship emerged between task uncertainty and team performance. Copyright © 2010 John Wiley & Sons, Ltd.