The illnesses of 40 patients with diagnoses of septicemia, cellulitis with bacteremia, pneumonia empyema, and meningitis caused by Streptococcus pyogenes, Group A, are described. Twenty-five of 27 patients (93%) without underlying disease survived, whereas only seven of 13 children (54%) with underlying disease survived. Nine of the 25 patients who were otherwise normal and who survived these infections had prolonged, complicated illnesses. Four of these patients, and one who died, had septicemia without a focus of infection at the time of admission. Streptococcus pyogenes, Group A, although very sensitive to penicillin G and other antibiotics, can cause both severe and rapidly progressive disease in children.