Nowadays, customized product development (CPD) is increasingly prevalent in business-to-business settings, which has motivated manufacturers into development approaches wherein the customer plays an active role. When the customer is merely viewed as a passive receiver of the customized product, the manufacturer won't be able to truly empathize with the customer and might lack important suggestions to create and improve the customized product. It is, after all, the customer that holds pertinent development information and/or expertise. Yet, customers are not always motivated to participate and often need to be convinced about the manufacturer's ability to develop customized products in a timely and cost-effective manner. Prior literature on interorganizational relationships suggests the use of formal control, i.e., process and/or output control, to fashion activities in line with expectations so that development goals can be attained. Thereupon, this study posits that the customer's use of such formal controls may stimulate customer participation in CPD. In addition, this study investigates whether manufacturers can indeed benefit from customer participation in CPD through improved new product performance. To accomplish the research objectives, survey-based and accounting data are collected on 63 collaborative CPD projects between a plastics manufacturer and its industrial customers. In conjunction with an add-on experimental study regarding the effect of formal control on customer participation, this study reveals that the customer's use of formal control significantly increases the level of customer participation in CPD. Additionally, this study confirms that customer participation positively impacts new product performance. Together, these results imply that letting the customer use process and/or output control helps the customer to believe more in the pursuit of CPD goals and successful product customization, thereby encouraging the customer to participate more actively in CPD. Besides, the findings imply that increased access to market and customer need-related information obtained through customer participation is indeed critical for successful CPD.