Bills of quantities have been documented to exist in some form or another as far back as when the Egyptian pyramids were being constructed. In spite of that age long history, the bills of quantities (BOQ) seems to be the most misinterpreted aspects of building documentation today. The pros and cons of the bills of quantities have been deliberated on for many years and have generated strongly held and differing views. Whilst this is recognised, the essence of this study is to evaluate the reliability of bills of quantities in building project procurement. The study was carried out using secondary data from some recently completed building projects within the Northern Ireland construction industry. Using secondary data from completed projects, the budgetary reliability of the bills of quantities in building project procurement was investigated. Data analysis was carried out using percentage deviation of final account figures from the bills of quantities. Further analyses were carried out using root mean square deviation and relative mean absolute deviation methods of analyses. Results showed that the budgetary reliability of the bills of quantities seems to vary depending on project types. Whilst a deviation of -3 to 4% was obtained on housing projects analysed, the deviation on educational projects was between -4 and 17% whist on commercial project, it came out to be between -20 and 20% and in the case of refurbishment projects, a deviation of between -11 and 37% was obtained. This seems to suggest that the more complex a project is, the less reliable it is to use the BOQ to guarantee cost certainty.