Physical activity is associated with reduced risk of several cancers, including aggressive prostate cancer. The mechanisms mediating the effects are not yet understood; among the candidates are modifications of endogenous hormone levels. Long-term exercise is known to reduce serum levels of growth stimulating hormones. In contrast, the endocrine effects ofacute endurance exercise include increased levels of mitogenic factors such as GH and IGF-1. Exercise serum from 9 out of 10 individuals had a growth inhibitory effect on LNCaP cells after 48 hours incubation (figure 1A and B) compared to incubation with the corresponding rest serum. Growth of NIH3T3 cells was increased by 5 individual exercise serums and reduced by 5 (figure 1C and D).