Plasma levels of pyridoxal phosphate (PLP) were determined in 20 patients with pulmonary tuberculosis before and after one week of drug therapy including isoniazid. At baseline, body mass index and PLP levels were reduced in 10 and 18 patients, respectively. After 7 days of therapy, PLP levels decreased (P < 0.001) in all but one subject who inadvertently received pyridoxine supplementation. The decreased PLP levels occurred despite a significant improvement in the acute phase response (increased albumin [P < 0.001] and reduced C-reactive protein levels [P < 0.01]). This study indicates the need for possible routine pyridoxine supplementation in patients with newly diagnosed tuberculosis.