OBJECTIVE To investigate the changes in intraprostatic blood flow during ProstaLund Feedback Treatment (PLFT) using positron emission tomography (PET). METHODS Three patients with bladder outlet obstruction caused by benign prostatic hyperplasia were treated with PLFT using the ProstaLund device, which has the ability to calculate the intraprostatic blood flow. Treatment was given for 1 hour. Five PET scans were done during each treatment to calculate the three-dimensional blood flow using [(15)O]H(2)O as the tracer. RESULTS The prostatic blood flow increased steeply at the beginning of the treatment in all three patients, by as much as 100% at 20 and 35 minutes. For patients 1 and 2, there was a fast decline in intraprostatic flow at the last scan (55 minutes), clearly seen as a large zone with circulation arrest centrally in the prostate. The intraprostatic temperature was <50 degrees C during the first 30 minutes but increased to 52 degrees to 60 degrees C during the second part of the treatment. Patient 3 had high blood flow during the entire treatment. A reduction of the blood flow was seen at the end of the treatment but not to the same extent as in the other two patients. The intraprostatic temperature did not exceed 49 degrees C for this patient. CONCLUSIONS The large variations seen in intraprostatic blood flow suggest that intraprostatic temperature monitoring is mandatory to optimize the treatment. The ProstaLund bioheat model calculates the change in intraprostatic blood flow accurately.