This fast abstract briefly discusses lessons learned in terms of how to overcome limitations of PDA devices, effectively capture requirements for mobile health application development and effectively reengineer a desktop application on PDA. The limitation of PDA devices includes: (1) small screen size, which limits text-based data entry, reporting and browsing; (2) limited storage space and slow processing, which requires effective and efficient coding generation; (3) security limitations and inadequate third party application support, which poses challenge for application development. The corresponding strategies addressing the above challenges are: (1) providing check box, radio button, drop down list, text field, combo box based data entry and minimize text-based data entry, such as text area; (2) allocating as much as possible data storage, communication and computation tasks to the desktop system and minimize the function PDA component needs to handle, store data in file instead of database on PDA; (3) carefully selecting the device and implementation platform that supports development. To maximize the usability of PDA-based health application, the solution to be implemented on PDA ideally should be data management tasks with minimum text-based data entry and high frequency of recording. Challenges and solutions for software reengineering from desktop system to PDA-based application is outlined based on one application developed for World Health Organisation. In this example, the complex task of communication among PDA-based application, desktop system and the existent desktop application EpiData is effectively handled through the utilisation of XML files. A dynamic tool 'Questionnaire Designer' provides a completely dynamic user interface generation tool that could easily be handled by end user. The design idea for this application sets up a model for mobile health application with adequate flexibility of handling changing data management needs of end users.