A randomized, double-blind, placebo-controlled study was conducted to evaluate if Lactobacillus helveticus Lafti L10 (Lallemand Health Solutions, Montreal, Que., Canada) supplementation during 14 weeks in winter can influence the duration, severity, and incidence of upper respiratory tract illness (URTI), as well as to monitor different immune parameters in the population of elite athletes. Before and after the treatment, cardiopulmonary testing and self-rated state of moods evaluation (by Profile of Mood States questionnaire) were performed and blood samples were collected. Thirty-nine elite athletes were randomized either to the placebo (n = 19) or the probiotic (n = 20) group. The probiotic group received L. helveticus Lafti L10, 2 × 10(10) Colony Forming Units. Lafti L10 significantly shortened the URTI episode duration (7.25 ± 2.90 vs. 10.64 ± 4.67 days, p = 0.047) and decreased the number of symptoms in the probiotic group (4.92 ± 1.96 vs. 6.91 ± 1.22, p = 0.035). Severity and incidence of URTI did not differ between the treatments. There were no significant changes in leukocyte subpopulation abundance, transforming growth factor-β serum levels, level of interleukin-10 secreted from peptidoglican stimulated peripheral blood mononuclear cells (PBMCs), interferon-γ level secreted from concanavalin A-stimulated PBMCs or viability/proliferation of PBMCs upon antigen stimulation. Group effect for CD4+/CD8+ ratio was significant (F[1,37] = 6.99, p = 0.020, η(2) = 0.350); this difference was not significant at baseline, but was evident after 14 weeks (p = 0.02). A significant interaction effect was noted for self-rated sense of vigor (F[1,37] = 11.76, p = 0.009, η(2) = 0.595). Self-rated sense of vigor increased in the probiotic group (18.5 ± 4.1 vs. 21.0 ± 2.6, p = 0.012). Probiotic strain Lafti L10 can be a beneficial nutritional supplement for the reduction of URTI length in elite athletes.