Requirements volatility is an important risk factor for software projects. Software measures can help in quantifying and predicting this risk. In this paper, we present an industrial case study that investigated measures of volatility for a medium size software project. The goal of the study was twofold: 1) to empirically validate a set of measures associated with the volatility of use case models (UCM); 2) to investigate the correlation between subjective and objective volatility. Measurement data was collected in retrospect for all use case models of the software project. In addition, we determined subjective volatility by interviewing stakeholders of the project. Our data analysis showed a high correlation between our measures of size of UCM and total number of changes, indicating that the measures of size of UCMs are good indicators of requirements volatility. No correlations was found between subjective and objective volatility. These results suggest that project managers at this company should measure their projects because of the risk to take wrong decisions based on their own and the developer´s perceptions.