The aims of the study were to [1] investigate the performance trends at the 'Zurich 12-h Swim' in Switzerland from 1996 to 2010; and [2] determine the gender difference in peak performance in ultraendurance swimming. In total, 113 male and 53 female swimmers competed in this indoor ultraendurance event while swimming in a heated pool. The number of male participants significantly increased (r² = 0.36, P = 0.04) over time while the participation of females remained unchanged (r² = 0.12, P = 0.26). In the age group < 19 years, the male swimmers achieved a significantly greater distance than the females (32.7 km vs. 21.9 km, respectively) (P < 0.05). In the older age groups (20-29 years, 30-39 years, 40-49 years, and 50-59 years, respectively) was no gender difference in swimming performances (P < 0.05). The best performance was achieved in the age groups 30-39 years and 40-49 years for both genders, respectively. The athletes in the age groups < 19 years, 20-29 years and 50-59 years in females were significantly slower than the athletes in the age group 30-39 years and 40-49 years (P < 0.05). For the males, the athletes in the age group 30-39 years were significantly faster than the athletes in the age group < 19 years. The annual best performance was not significantly different between males and females (38.3 ± 2.9 km vs. 34.4 ± 8.2 km, respectively) (P < 0.05). The best male and female swim performances remained unchanged across the years. Females are able to achieve a similar swim performance in an indoor ultra-endurance swim event of ~40 km. Further studies are needed to investigate whether females are able to achieve similar or even better performances than males in openwater ultra-swimming events such as 'Channel Swimming'.