Reanalysis and retesting of modified software is a major factor contributing to the high cost of maintaining evolving software systems. To reduce this cost, changes must be incorporated into a system and tested in an efficient way. We describe an incremental testing system, which is based on data-flow testing, to aid in unit testing during the maintenance stage. The system is designed for use in a programming environment and thus it shares common information and techniques with tools typically found in programming environments. In response to a change in a module, the incremental tester reuses the analysis and test cases from the previous testing session. The analysis is incrementally updated to determine areas of the program that must be retested. From the updates, the analyzer determines which test cases must be rerun, which test cases may be eliminated, and whether the test cases are sufficient for testing the changed program according to the data-flow coverage criterion.