This paper describes how the use of bicycle helmets in Sweden has changed for different categories of cyclists from 1988 to 2002, and it also estimates future trends in voluntary wearing of bicycle helmets up to the year 2010. Observational studies of the use of bicycle helmets were conducted once a year from 1988 to 2002 at 157 sites in 21 cities. The subjects observed were children cycling to school (average n = 5471/year) and in their free time (average n = 2191/year), and adults cycling to workplaces and on public bike paths (average n = 29 368/year). The general trend in helmet use from 1988 to 2002 was determined by linear regression analysis, and the results were also employed to estimate future helmet wearing for the period 2003-2010. Differences in helmet use according to gender and size of city were analysed by chi-square tests. From 1988 to 2002, all categories of cyclists showed an upward trend in helmet use (p