The paper presents a case study investigating critical factors affecting production ramp-up. The study was performed in an assembly line of a medium-size Swedish manufacturing company. The context and performance during ramp-up was analyzed. Empirical findings indicate that ramp-up performance depends to a large extent on how earlier phases of the product realization process have been carried out. Specifically, supplier choice, relationships with suppliers, involvement of personal, verification of the match between product and process, resource allocation for critical processes, as well as training of assembly personnel seem to have major impact on performance during production ramp-up.