The objective of this roadside observational study was to monitor helmet wearing among motorcycle riders and passengers in three provinces (Yen Bai, Da Nang and Binh Duong) in the Socialist Republic of Viet Nam, before and after a mandatory helmet law took effect on 15 December 2007. A total of 665,428 motorcycle riders and passengers were observed between November 2007 and February 2011 at 45 randomly selected sites covering the entire road network. Across all locations and time periods, correct helmet wearing averaged 40.1% before the law and 92.5% after; however, there were significant differences between time points and locations. The Viet Nam Government's decision to require all motorcycle riders and passengers to wear helmets has been thoroughly implemented nation wide and the results show that high wearing has been sustained. Further study is required on how high helmet wearing has and will translate into a reduction in motorcycle head injuries; however, Viet Nam's motorcycle helmet legislation should be seen as an important policy example for other low- and middle-income countries with a high utilization of motorcycles for personal transport.