Radiosondes ascending near to mountainous areas in the United Kingdom occasionally encounter very large-amplitude gravity waves in the stratosphere. These ascents are characterized by large perturbations in the temperature (~5 K) and vertical velocity (2--10 m s-1) on vertical scales of about 2 km. Some of the profiles suggest a large vertical flux of horizontal momentum (order 1 N m-2). Examples of these waves are illustrated, and the situations in which they are observed are described. Simple linear wave theory has been used to diagnose many of the important wave properties.