ContextThe analysis and selection of requirements are important parts of any release planning process. Previous studies on release planning have focused on plan-driven optimization models. Unfortunately, solving the release planning problem mechanistically is difficult in an agile development context. ObjectiveWe describe how a release planning method was employed in two case projects in F-Secure, a large Finnish software company. We identify the benefits which the projects gained from the method, and analyze challenges in the cases and improvements made to the method during the case projects. MethodWe observed five release planning events and four retrospectives and we conducted surveys in the first two events. We conducted six post-project interviews. We conjoined the observation notes, survey results and interviews and analyzed them qualitatively and quantitatively. ResultsThe focal point of the method was release planning events where the whole project organization gathered to plan the next release. The planning was conducted by the development teams in close collaboration with each other and with the other stakeholders. We identified ten benefits which included improved communication, transparency, dependency management and decision making. We identified nine challenges which included the lacking preparation and prioritization of requirements, unrealistic schedules, insufficient architectural planning and lacking agile mindset. The biggest improvements to the method were the introduction of frequent status checks and a big visible planning status board. ConclusionThe release planning method ameliorated many difficult characteristics of the release planning problem but its efficiency was negatively affected by the performing organization that was in transition from a plan-driven to an agile development mindset. Even in this case the benefits clearly outweighed the challenges and the method enabled the early identification of the issues in the project.