We discuss the consequences of undetected lies for professional relationships. Lying is common in various professional contexts and typically goes undetected. However, recent research on deception in negotiation reveals that, despite potential economic benefits, undetected lies have hidden relational costs. In our studies, lies were designed to enable lucrative economic deals, and lies typically remained hidden from unsuspecting targets. Nevertheless, lying undermined negotiators’ satisfaction with their own deals and reduced their desire to interact with their counterpart again. We conclude that despite the potential short-term benefits of deception in negotiation, encouraging honesty in the workplace is crucial for building high-quality relationships and promoting employee engagement.