The Mayo Clinic says Rep. Jesse Jackson Jr., an Illinois Democrat, is being treated for bipolar depression at its clinic in Rochester, Minn.
"Congressman Jackson is responding well to the treatment and regaining his strength," Mayo Clinic said in a statement.
Jackson Jr.'s condition has forced him to take a leave from Washington since June 10. His whereabouts have led to widespread speculation and calls for him to release more details of his condition.
The Mayo Clinic said Jackson is suffering from bipolar II, "a treatable condition that affects parts of the brain controlling emotion, thought and drive and is most likely caused by a complex set of genetic and environmental factors."
Bipolar disorder is characterized by high and low mood cycles. WebMd describes bipolar II disorder as being similar to bipolar I, except that the "up" moods "never reach full-on mania."
There is no word on what this means for his political career. Jackson's spokesman refused to comment when the AP inquired.
The AP quoted an aide last week saying that Jackson was expecting to return "within a matter of weeks."