Scientists have derived a new value for the Avogadro constant using X-ray measurements on a single crystal of silicon. The new value measured by Peter Becker and colleagues at the PTB standards laboratory in Braunschweig, Germany, and the Institute for Reference Materials and Measurements in Geel, Belgium, is part of an international effort to redefine the kilogram in terms of atomic and fundamental constants (P Becker et al. 2003 Metrologia 40 271). However, the value measured by Becker and colleagues - 6.022 135 3 x 10²³ mol^-1 - differs significantly from the value recommended by the Committee on Data for Science and Technology (CODATA).