The intensive treatments seem to work best for anxiety-related disorders. They usually consist of C.B.T., in which patients repeatedly expose themselves to the very situations they fear.

Supporters of the approach said that while it may involve a similar number of total hours as weekly therapy, relief is quicker. Thomas H. Ollendick, a psychology professor at Virginia Tech, who helped pioneer a one-day treatment for phobias and has studied a one-week treatment for O.C.D., said this can be crucial for people whose illnesses are preventing them from attending school or work.

And with concentrated treatment, Dr. Ollendick said, “you don’t have a week in between to unlearn what you learned in the session or have additional experiences that can lead you to think, ‘Oh, I better be afraid.’”

The concentrated format allows therapists to deliver evidence-based treatment to more people, since it’s easier for patients who live in places without access to high-quality therapy to travel for a one- or two-week program, said Donna B. Pincus, director of the Child and Adolescent Fear and Anxiety Treatment Program at Boston University, which runs five- to eight-day intensive treatment programs for panic disorder, separation anxiety disorder and phobias.

Even patients who live nearby may find it easier to take off a week of work or plan treatment during a school break, rather than deal with the logistics of weekly therapy.

“People are pulled out of their everyday lives for two weeks — they are not dealing with work and spouses and kids,” said Barbara O. Rothbaum, a professor of psychiatry and behavioral sciences at the Emory University School of Medicine. “It really is a kind of a bubble for them to do this work.”