It seems clear that there is no simple solution to improved predictability of software development projects. Over several decades various aspects of software development and their relationship to estimation accuracy have been investigated. This paper focus on one such relationship; the clients' impact on estimation accuracy in software development projects. Factors contributing to overruns as well as factors preventing overruns are investigated. Based on a literature review and a survey of 300 software professionals we find that: 1) It is software professionals' perception that clients impact estimation accuracy. Changed and new requirements are perceived as the clients' most frequent contribution to overruns, while overruns are prevented by the availability of competent clients and capable decision makers. 2) Survey results should not be used in estimation accuracy improvement initiatives without further analysis. Surveys typically identify direct and project specific causes for overruns, while substantial improvement is only possible when the underlying causes are understood. Keywords: software estimation, survey, client-vendor relationship, overrun causes