We report on the application of the use-case modeling technique to identify and specify the user requirements of the MammoGrid project in an incremental and controlled iterative approach. Modeling has been carried out in close collaboration with clinicians and radiologists with no prior experience of use cases. The study reveals the advantages and limitations of applying this technique to requirements specification in the domains of breast cancer screening and mammography research, with implications for medical imaging more generally. In addition, this research has shown a return on investment in use-case modeling in shorter gaps between phases of the requirements engineering process. The qualitative result of this analysis leads us to propose that a use-case modeling approach may result in reducing the cycle of the requirements engineering process for medical imaging.