The deterioration of performance caused by aging was examined in 3,469 swimmers divided into 9 age groups and 4 styles of swimming. Comparisons using cross and longitudinal sections show that older swimmers become progressively slower, and the reduction of speed in all styles of swimming follows a quadratic trend. The losses of efficiency increase with age group, being by far the most in back-stroke and the least in crawl swimming. The combined longitudinal and cross sectional comparisons confirm this result, but not without any exception, for very often no statistically significant differences could be recognized between adjacent age groups. The interindividual comparison shows that it is also possible to maintain or even improve the efficiency over 10 to 12 years, even in the fifth or sixth decade of age, if the intensity and volume of training can be increased. The curve comprising reductions in the later stages of life may additionally be caused by the smaller number of persons per group and the deficient homogeneity within these groups. In addition to this, the tendency towards higher losses in older age groups is not significant, if only the best competitors of each age group are compared.