Although moderate exercise is beneficial for the human body and its immune system, exhaustive ultra-endurance performance in cold conditions might be harmful. The aim of this study was to examine the effect of a 100 km ultra-marathon under cold conditions (temperatures from minus 1°C to +1°C) on selected immunological, biochemical and hematological parameters. Participants were 15 runners (12 men and three women, age 40.3 ± 9.7 years, body mass 67.3 ± 9.0 kg and body height 1.74 ± 0.10 m, mean ± standard deviation). Leukocytes statistically increased and, particularly, the number of leucocytes doubled in seven out of 15 athletes. Immature neutrophils, mature neutrophils and monocytes statistically increased, whereas lymphocytes and eosinophils did not change. IgG statistically increased, but IgA and IgM remained unchanged.