A microscope with band-pass filters was used to measure the optical-density distribution of GafChromic films irradiated with multi-slit microbeam X-rays. The planar width was 25 mu m, and the center-to-center distance was 200 pm. The peak and valley dose rates in air were found to be 120 and 0.7 Gy/s, respectively. In a poly methylmethacrylate phantom, the peak-to-valley dose ratio decreased to 80 at a 1-mm depth. Doses calculated with the PENELOPE code agreed with those around the peak but became smaller in the valley. (C) 2008 Elsevier Ltd. All rights reserved.