This article reports how we used grounded theory to study whether and how system developers use Information Systems Development methodologies in practice. The work is founded on a case study carried out in a large software company. The data collection was based on semi-structured qualitative interviews with staff and it was supplemented by literature and documents studies. As our intention was to study how practice behaves instead of investigating if practice behaves in a specific way we had to use an open research method. With the collected data we therefore performed an analysis based on the grounded theory methodology. Grounded theory describes a general way to search relevant topics and relations through three sequential steps, but gives little concrete advice of how to actually perform a grounded theory analysis. We here provide such a detailed description of our approach, and discuss its advantages as well as its potential problems.