This study examines whether assurance on the voluntary provision of nonfinancial performance indicators affects the stock price estimates of a group of sophisticated financial report users. We conducted an experiment where participants were provided with a case study containing excerpts from a hypothetical company's annual report. Nonfinancial performance and assurance were manipulated in a 2 (positive and negative nonfinancial performance indicators) ×2 (assurance and no assurance) +1 (control condition) between-subjects design. After reading the case materials, the participants indicated whether they believed the company's stock price would increase or decrease based on the information provided. As expected, we found that the nonfinancial performance indicators had a significant effect on stock price estimates. In addition, consistent with attribution theory, an assurance report on the voluntarily disclosed nonfinancial performance indicators only had a significant effect on stock price estimates when the nonfinancial performance indicators were positive, suggesting that the value of assurance is context-specific. Our research contributes to the discussion on the value of expanded assurance services and also on the value of enhanced corporate disclosure.