During one of the hottest summers in recorded history comes a new way to never let them see you sweat: Microwave your sweat glands.

The procedure, called miraDry, from Miramar Labs Inc. in Sunnyvale, Calif., was approved by the Food and Drug Administration last year and became available in January to treat hyperhidrosis, or excessive sweating. The procedure uses microwave energy to destroy many of the 22,000 to 30,000 sweat glands in the armpits, Miramar says. Unlike other nonsurgical treatments for excessive sweating, it...