Design may be seen as one of several key factors contributing to new product development, along with research and development, marketing, manufacturing, purchasing, etc. More and more, creative design comes to the fore, and many companies believe that superior design will be the key to winning customers. It has the ability to create corporate distinctiveness and also possesses the potential to give a product an individual or new look. Furthermore, the model of open innovation suggests that firms can and should use external and internal knowledge flows in order to create valuable ideas, and also internal and external paths to the market. Also, in the design process, a common trend toward external design skills has emerged in recent years. Due to cost and control factors, firms are increasingly outsourcing design activities. By using a sample of Belgian companies, this paper explores the contribution of design activities to product market performance. While there is mounting evidence that design can be seen as a strategic tool to successfully spur sales of new product developments at the firm level, the topic of design innovation has not yet been linked to the open innovation concept. In this paper, it is empirically tested whether design activities conducted in house differ in their contribution to new product sales from externally acquired design. So, do design activities that have been developed only with internal resources lead to a greater success than those that have been carried out with external sources of knowledge? Using a large cross-section of manufacturing and service firms, the effects on sales of products new to the market and of imitations or significantly improved products of the firm are investigated. At first glance, the findings indicate that externally acquired design is not superior to in-house design activities: the results show that only design activities that are mainly conducted with internal knowledge sources play a crucial role regarding the product innovation's success with market novelties. Design conducted in collaboration with external partners, however, has no significant influence. This is not the case for imitations, that is, products only new to the firm. Their success is also influenced by design activities developed with external collaborators. This effect is robust for several modifications of the model specification. In contrast to earlier literature on new technological developments, this paper argues that external design may not affect the sales of market novelties as the 'market news' may spill over quickly to rivals through common suppliers including external designers. [ABSTRACT FROM AUTHOR]