On May 1, 2001, a tractor-semi-trailer that had horizontally mounted cylinders filled with compressed hydrogen struck a pickup truck near Ramona, Oklahoma. During the accident, some of its cylinders, valves, piping, and fittings were damaged and released hydrogen. The safety issues discussed in this report are the adequacy of Federal requirements for protecting cylinders that are horizontally mounted on semitrailers and the valves, piping, and fittings of the cylinders during rollover accidents and the adequacy of the information in the North American Emergency Response Guidebook about compressed hydrogen. The NTSB made recommendations to the Research and Special Programs Administration.