Michael was to lead concerts on February 24 and 25 and March 3 and 4. David Atherton has replaced him for the first weekend of concerts; music director designate Michael Christie will step in for the second.
In a statement, the orchestra said that Michael's doctors had advised him not to travel. Michael was diagnosed with aplastic anemia, a rare blood disorder, in 1999.
"Our hearts go out to Maestro Hermann Michael at this time," said Maryellen H. Gleason, the orchestra's president and CEO. "We know the decision not to return to Phoenix this year was very difficult for him, but we also understand the seriousness of his medical condition and we wish him the best of health going forward. We send our prayers to Hermann and his wife Bigi, and are hopeful that he will be able to join us as conductor laureate during the 2005-06 season."