X-ray powder diffraction data, unit-cell parameters, and space group for letrozole, C17H11N5, are reported [a = 7.034(0) Å, b = 16.177(5) Å, c = 13.411(3) Å, α = γ = 90°, β = 105.71(9)°, unit-cell volume V = 1469.0(3) Å3, Z = 4, and space-group P21/c]. All measured lines were indexed and are consistent with the P21/c space group. No detectable impurity was observed.