Summary.One case of paroxysmal tachycardia at a 3 weeks old boy is described.Symptoms: Pronounced tachycardia, bad general state, cyanosis, dyspnoea, enlargement of the liver. The diagnosis is made with the help of electrocardiogram. The therapy has been neo‐stigmin injections, and during the free intervals, neostigmin per os as far as could be seen with good effect. The prognosis for this specific case seemed to be good with this treatment, but the danger of relapse makes the therapy a bit uncertain. CHRISTENSEN in 1944 had made a synthesis from the literature of 37 cases. The prognosis seems also in these cases to be good, when it has not been the question of organic heart diseases. The very bad general state makes it plausible that some cases of death of infants with uncertain diagnosis would be blamed on non‐diagnosticated attacks of paroxysmal tachycardia. The treatment has earlier consisted of digitalis or strophanticus. However, the effect has not been reliable. Mecholyl also was used. This drug seems to have sure effect but to give more toxic effect than the neostigmin, which was used in this case.