The use of experience data is frequently suggested as a mean to increase software effort estimation accuracy. However, there has been done limited empirical research on the subject. To investigate the effect of using experience data, we conducted a study on eighteen of the latest projects of a software contractor. Quantitative project data was collected, and project managers interviewed, on several issues related to estimates, key project properties, and project outcome. It was found that in projects where experience data was utilized in the estimation process, they experienced a lesser magnitude of effort overruns. The use of a checklist also appeared to increase estimation accuracy. However, the utilization of an estimation model in the estimation process appeared not to have any impact on the estimation accuracy.