Knowledge of useful fistula flow (UFF), the maximum blood flow available using twin needles within acceptable pressure limits and without recirculation, is essential for the optimal management of patients receiving maintenance hemodialysis or hemofiltration. A technique for the measurement of UFF employing thermal dilution for the detection of recirculation has been developed. Using this technique, 94 studies were carried out in 69 forearm fistulae. UFF exceeded 400 ml/min in 45 fistulae. In these satisfactory fistulae a normal range was defined for basal intrafistula pressures (60 mmHg being the approximate upper limit of normal). Analysis of arterial and venous line pressure recordings with increasing extracorporeal blood flow and knowledge of the presence or absence of recirculation allowed us to define the functional problem in unsatisfactory fistulae. The functional diagnosis was supported in 16 of 24 fistulae by angiography or surgery or both. In all but two of the remainder, satisfactory UFF was obtained by repositioning the patients' needling sites. Fistulae were divided into different clinical groups. Of 35 fistulae which were thought to be clinically acceptable, 7 were found to be unsatisfactory. In 17 fistulae in patients with poor biochemical control, recirculation was detected in 8. Of 11 fistulae reported to produce poor flows on dialysis, 4 had UFF above 400 ml/min. Of 6 fistulae in patients experiencing needling difficulties, 5 had satisfactory UFF. These studies which take only a few minutes and can be carried out immediately preceding a routine dialysis session not only identify unsatisfactory fistulae, but yield valuable diagnostic information in these cases. This has reduced dependence on angiography and has led to more careful selection of patients for surgery.