This study investigated the change in sex differences across years in ultra-distance swimming performances at the 36-km 'Maratona del Golfo Capri-Napoli' race held from 1954 to 2013. Changes in swimming performance of 662 men and 228 women over the 59-year period were investigated using linear, non-linear and hierarchical regression analyses. Race times of the annual fastest swimmers decreased linearly for women from 731 min to 391 min (r 2 = 0.60, p < 0.0001) and for men from 600 min to 373 min (r 2 = 0.30, p < 0.0001). Race times of the annual top three swimmers decreased linearly between 1963 and 2013 for women from 736.8 ± 78.4 min to 396.6 ± 4.5 min (r 2 = 0.58, p < 0.0001) and for men from 627.1 ± 34.5 min to 374.1 ± 0.3 min (r 2 = 0.42, p < 0.0001). The sex difference in performance for the annual fastest decreased linearly from 39.2% (1955) to 4.7% (2013) (r 2 = 0.33, p < 0.0001). For the annual three fastest competitors, the sex difference in performance decreased linearly from 38.2 ± 14.0% (1963) to 6.0 ± 1.0% (2013) (r 2 = 0.43, p < 0.0001). In conclusion, ultra-distance swimmers improved their performance at the 'Maratona del Golfo Capri-Napoli' over the last ~60 years and the fastest women reduced the gap with the fastest men linearly from ~40% to ~5-6%. The linear change in both race times and sex differences may suggest that women will be able to achieve men's performance or even to outperform men in the near future in an open-water ultra-distance swimming event such as the 'Maratona del Golfo Capri-Napoli'.