Participation and performance trends for age group marathoners have been investigated for large city marathons such as the 'New York City Marathon' but not for mountain marathons. This study investigated participation and trends in performance and sex difference in the mountain marathon 'Jungfrau Marathon' held in Switzerland from 2000 to 2014 using single and mixed effects regression analyses. Results were compared to a city marathon (Lausanne Marathon) also held in Switzerland during the same period. Sex difference was calculated using the equation ([race time in women] - [race time in men]/[race time in men] × 100). Changes in sex differences across calendar years and were investigated using linear regression models. In 'Jungfrau Marathon', participation in all female and male age groups increased with exception of women in age groups 18-24 and men in age groups 30-34, 40-44 and 60-64 years where participation remained unchanged. In 'Lausanne Marathon', participation increased in women in age groups 30-34 to 40-44 years. In men, participation increased in age groups 25-29 to 44-44 years and 50-54 years. In 'Jungfrau Marathon' runners became slower across years in age groups 18-24 to 70-74 years. In 'Lausanne Marathon', runners became slower across years in age groups 18-24 and 30-34 to 65-69 years, but not for 25-29, 70-74 and 75-79 years. In 'Jungfrau Marathon', sex difference increased in age groups 25-29 (from 4 to 10 %) and 60-64 years (from 3 to 8 %) but decreased in age group 40-44 years (from 12 to 6 %). In 'Lausanne Marathon', the sex difference showed no changes. In summary, participation increased in most female and male age groups but performance decreased in most age groups for both the mountain marathon 'Jungfrau Marathon' and the city marathon 'Lausanne Marathon'. The sex differences were lower in the 'Jungfrau Marathon' (~6-7 %) compared to the 'Lausanne Marathon' where the sex difference was ~10-12 % from age groups 18-24 to 55-59 years. These unexpected findings might be a typical Swiss phenomenon. Future studies need to investigate whether this trend can also be found in other endurance sports events held in Switzerland and other mountain marathons held in other countries.