In Michigan, legionellosis cases are reportable through the Michigan Disease Surveillance System (MDSS), a web-based database. The objectives of this study were to evaluate Michigan's legionellosis surveillance system and determine whether diagnostic methods influenced case distribution. Michigan's legionellosis surveillance system was evaluated using the 2001 MMWR surveillance system evaluation guidelines focusing on data quality, timeliness, and sensitivity. Additional analyses were performed on the diagnostic test data. The system showed moderate sensitivity, and reporting times over the 24-hour state requirements. Data completeness improved 20% over time. Decreasing culture diagnoses make linking cases to environmental sources in an outbreak investigation challenging.