The problem of cost overrun in construction projects is considered to be one of the most important issues in the project life cycle. Project efficiency is often defined exactly by the cost overruns, and the effective project management is crucial for successful project closure. The phenomenon of cost overruns can occur on construction projects of all kinds, regardless the size and complexity. The changes associated with cost overruns can seriously devastate budget to the point where the project could not be finished with required quality standards, or it could even have to be abandoned. The client expects that the final cost of the project will not exceed contracted value. Also, it is expected that the execution of the project would be at required time frame and required quality assurance. An awareness of factors that can cause cost escalations enables to identify potential problems of cost overrun negative effects and to take appropriate action to eliminate, or at least minimize them. This study performs an analysis of the cost overrun factors in construction projects. A study was undertaken to investigate the correlation between committed cost and total cost of the project depending on cost category and probability of cost overruns depending on the type of construction work. The special focus is given on phenomenon of „Optimism bias“. The first step of analysis was collecting the data and creating a database for making a statistical model. Linear statistical regression and normal distribution are adopted for estimating the relationships among mentioned variables. After reviewing the results of analysis, recommendations are given to reduce and prevent cost overruns.