Mike Brandon, 31, from Bristol, is being treated in Philadelphia after his British consultant said all NHS routes were "exhausted".
Kate Brandon said it had been "by far the hardest three weeks of our lives."
She said her husband had spent the last week in intensive care but was now back on the "wonderful oncology unit".
Mr Brandon was diagnosed with acute lymphoblastic leukaemia two years ago.
In May, his wife raised enough money using a social media campaign to fund his taking part  in "revolutionary" cancer trials at the University of Pennsylvania.
Mrs Brandon said: "We were told that paradoxically the sicker that people become during this treatment, the more likely they are to have a good outcome.
"This knowledge has helped carry us through this excruciatingly difficult time. Boy, did our boy get sick."
Mr Brandon said: "There are plenty of things about a leukaemia diagnosis that are tough: the constant worry about things that may never happen, the strain of the things that have definitely happened, and the stress it places on those closest to me.
"When my leukaemia relapsed in March, we were told that it was time to plan our last days together, but Kate point blank refused. We would fight on, and we have."
A man with a rare type of leukaemia whose wife raised £400,000 to cover his medical costs in the USA is "well under way" with his cancer treatment.