Some people worry that Whitney Houston—who was absent from several recent planned appearances—is behaving erratically. Her supporters say there's nothing to worry about. Houston's performance at the Arista Records 25th-anniversary party offered evidence for both sides.

Vocally, Houston—introduced by Danny Glover as "the greatest singer in the world today"—was remarkable, filling L.A.'s cavernous Shrine Auditorium and offering a thrilling tour of her five-octave range. But for those concerned about Houston's health, there were unsettling moments. When she took the stage toward evening's end to sing a 20-minute medley of her hits, the singer stumbled for a moment in her long gown. At times she spoke rather than sang lyrics, and in the middle of "I Will Always Love You," she stopped cold and asked for a glass of water.

Even so, she was the darling of a crowd that included a parade of performing Arista-cats—from Barry Manilow to Carlos Santana. Backstage after the show, with Houston, 36, nowhere to be seen, the vibe was all love and support. "She's at the top of her game," said Arista founder Clive Davis, the evening's honoree. "She's a professional." Dionne Warwick, Houston's cousin, declined to discuss the singer's problems except to say, "She'll be fine."