Prediction Markets are a relatively novel type of Group Decision Support System (GDSS) which use social collaboration via a market mechanism to elicit and aggregate information from large numbers of individuals. While the literature recognises their potential as decision support tools, it also notes several issues that give rise to concern regarding their utility in an organisational setting. One concern is the possibility that prediction markets may be subject to malicious manipulation. This paper presents a field experiment which examines the effect of such manipulations on prediction market performance.