Three patients with Mycoplasma pneumoniae infection, documented by CF antibody titers greater than 128, developed severe extrapulmonary manifestations. Two of them had a Stevens-Johnson syndrome seven and two days after the beginning of respiratory symptoms and the third a demyelinating polyradicyloneuropathy three weeks later. One of the patients with Stevens-Johnson syndrome and serious respiratory involvement also had hemolytic anemia associated with a cold agglutinins titer of 128. Authors determined lymphocytes subpopulations in one of the cases of Stevens-Johnson syndrome and in the patient with polyradiculoneuropathy. In both of them they found a reduced number of T-helper lymphocytes, and in the patient with Guillain-Barré syndrome an increase in suppressor T cells was also demonstrated.