The losses of amino acids and related compounds have been studied during hemodialysis of patients with the Kiil dialyzer and the Dow Hollow Fiber Artificial Kidney (HFAK). There was a linear relationship with close correlation for both direct dialysance (D<sub>d</sub>) and indirect dialysance (D<sub>p</sub>) between the two dialyzers, the HFAK being 31–46% more efficient than the Kiil dialyzer in removing these solutes. The volume from which the amino acids were extracted was found to be the plasma alone, since the ratio D<sub>p</sub>/D<sub>d</sub> was close to 1.00 for both dialyzers. A good correlation was found for both dialyzers between molecular weight and dialysance (r = 0.53,0.57) but the correlation was even closer between the diffusion coefficient in water and the dialysance of the solutes (r = 0.86, 0.89). The absolute losses of the amino acids during dialysis for short periods in the fasting state were extrapolated to 8 h, the usual duration of a dialysis. For the Kiil dialyzer extrapolated losses were sizeable (as percentage of minimal daily requirements) for only three essential amino acids: valine (36%), lysine (44%) and threonine (48%). With the HFAK the losses of all essential amino acids except phenylalanine and methionine were sizeable, the highest being for valine (59%), lysine (59%) and threonine (108%). The possible significance of these losses to patients on chronic dialysis are discussed.