The transmission and clinical manifestations of infection with Mycoplasma pneumoniae were studied in 114 families in which there was a case of atypical pneumonia. M pneumoniae was isolated from a throat culture of the index pneumonia patient in 36 of these families. Transmission to other family members occurred in 23 of the 36 families. In these 23 families, 84% of the children and 41% of the adults were infected. The index cases are included. Of the total 59 patients with family-contact infections, 42 had lower-respiratory-tract symptoms, 6 had pharyngitis alone, 9 (all children) were asymptomatic, and 2 had probably unrelated symptoms. The time intervals between cases within a family suggested a median incubation period of 23 days. Treatment with tetracycline seemed neither to cure the symptoms completely nor to abolish the carrier state that often lasted one to three months.