There is growing interest on haematological changes following elite and recreation sports. However, no information is available on leucocyte variations after an intermediate-distance run. Complete blood cell count was assessed on 17 trained, middle-aged males before a 21-km half-marathon, at the end and 3, 6, 24 h thereafter. Statistically significant variations by one-way analysis of variance were observed for all the parameters tested. Haematocrit, haemoglobin and red blood cells and platelet count increased significantly by the end of the run and returned to pre-run values 3 h thereafter. White blood cells, neutrophils, monocytes and basophils counts increased after the run, reached the peak at 3 h and returned to the baseline after 24 h. Conversely, the eosinophils count significantly decreased after the run, reached a nadir at 3 h and slowly returned to pre-run values at 24 h. The lymphocytes count significantly increased after the run, decreased 3 h thereafter and returned to pre-run values after 6 h. These results show that an intermediate-distance run, which is a popular form of recreational sports worldwide, is associated with transitory leucocytosis, neutrophilia, monocytosis and basophilia, but it also induces acute eosinopenia, all changes being completely reversed 24 h thereafter.