Several studies have shown an association between the hemodialysis session length (the t of Kt or Kt/V) and favorable outcomes for patients on maintenance hemodialysis. In a single randomized controlled trial that systematically varied hemodialysis session length, shorter session length was associated with an increased risk for morbidity and mortality, independent of the time-averaged concentration of urea. Observational studies of dialysis session length have yielded conflicting results, although virtually all studies have confounded hemodialysis session length with hemodialysis efficiency or dose. Limited observational data from nocturnal hemodialysis programs more strongly suggest an independent beneficial effect of longer session length. In aggregate, data on the effects of hemodialysis session length are inconclusive. Future studies should evaluate hemodialysis session length independent of efficiency, and should consider the evaluation of dose by using other clearance parameters and the adequacy of ultrafiltration in addition to solute kinetics.