Software vendors increasingly aim to apply the Software-as-a-Service (SaaS) delivery model instead of the traditional on-premise model. Platforms-as-a-Service (PaaS), such as Google App Engine and Windows Azure, deliver a computing platform and solution stack as a service, but they also aim to facilitate the development of cloud applications (SaaS). Such PaaS offerings should enable third parties to build and deliver multi-tenant SaaS applications while shielding the complexity of the underpinning middleware and infrastructure. This paper compares, on the basis of a practical case study, three different and representative PaaS platforms with respect to their support for SaaS application development. We have reengineered an on-premise enterprise application into a SaaS application and we have subsequently deployed it in three PaaS-based cloud environments. We have investigated the following qualities of the PaaS platforms from the perspective of SaaS development: portability of the application code base, available support for creating and managing multi-tenant-aware applications, and quality of the tool support.