A technique called Brillouin optical-fiber time-domain analysis (BOTDA) is described for evaluating fiber loss. Two laser beams, a pulsed light (pump) and CW light (probe), counterpropagate in the test fiber. When the CW light frequency coincides with the Stokes frequency of the pulsed light, CW light is amplified thorugh the Brillouin interaction and then detected. Brillouin amplification of CW light can be observed during a time interval of 2L/v, where L is the fiber length and v is the light velocity in the fiber. The optical power increase is proportional to fiber attenuation, which allows evaluation of the fiber loss. Results of BOTDA and optical time-domain reflectometry, using the same pulsed laser and detecting optics, are compared.