The crystal structure of pimecrolimus Form B has been solved and refined using synchrotron X-ray powder diffraction data optimized density functional techniques. Pimecrolimus crystallizes in the space group P 2 1 (#4) with a = 15.28864(7), b 13.31111(4), c 10.95529(5) Å, β 96.1542(3)°, V 2216.649(9) Å 3 , Z 2. Although there are an intramolecular six-ring hydrogen bond some larger chain ring pa...