Flame synthesis is one of the most versatile and promising technologies for large-scale production of nanoscale materials(1-3). Pyrolysis has recently been shown to be a useful route for the production of single-walled nanotubes(4), quantum dots(5) and a wide variety of nanostructured ceramic oxides for catalysis(6) and electrochemical applications(7). An understanding of the mechanisms of nanostructural growth in flames has been hampered by a lack of direct observations of particle growth(8-21), owing to high temperatures (2,000 K), rapid kinetics (submillisecond scale), dilute growth conditions (10-6 volume fraction) and optical emission of synthetic flames. Here we report the first successful in situ study of nanoparticle growth in a flame using synchrotron X-ray scattering. The results indicate that simple growth models, first derived for colloidal synthesis(22), can be used to facilitate our understanding of flame synthesis. Further, the results indicate the feasibility of studies of nanometre-scale aerosols of toxicological(23) and environmental(24) concern.