The influence of a high magnetic field on the band structure and optical conductivity of FeSi has been studied by the ab initio full-potential LMTO-PW and relativistic LMTO-ASA methods with an applied field included. It is found that the applied external magnetic field induces a metallic magnetic state through a first-order metamagnetic transition, i.e. at a critical magnetic field of about there is a large steplike increase in the calculated magnetization, with a magnetic moment of . The magnetized state persists while the field is gradually reduced from above the metamagnetic transition (a hysteresis effect). The calculated optical conductivity (OC) at the critical field differs substantially from the zero-field OC in the infrared region.