Described in 1890, Uhthoff's symptom corresponds to a transient blurred vision provoked by physical exercise or an increase of temperature. It is a frequent symptom occurring during the course of multiple sclerosis.We report here four cases of patients presenting isolated Uhthoff phenomenon preceding multiple sclerosis by several years.These four patients presented transient neurological symptoms induced by intensive sporting activity for 1 to 6 years before diagnosis of multiple sclerosis. These symptoms were often visual but sometimes motor or sensorial. All symptoms appeared above a certain threshold specific to each patient, after 15 to 30minutes of intense physical exercise (bike, running or handball) and all disappeared after a few minutes to one hour rest with full recovery to baseline.Uhthoff's phenomenon is explained by a conduction block. It is due to axonal demyelization that provokes a reorganization of sodium channels induced by a decrease in a safety factor highly sensitive to temperature, or by release of soluble blocking substances (oxide nitric or cytokines).Without being specific, this symptom is strongly suggestive of this disease. Our case reports confirm the existence of "infraclinic multiple sclerosis".