The transmission rate and the clinical and pulmonary manifestations of serologically proven mycoplasma infections were reviewed in 34 patients from 11 families consisting of 30 children and adolescents aged 1 to 18 years, and four parents aged 30 to 41 years (total number of family members = 59:37 children, 22 adults). Twenty-seven of the 37 children had pulmonary involvement (73%). The total infection rate was 58%, and the infection rate in children 81%. The roentgenologic findings in children with pneumonia showed no pathognomonic features. The most common X-ray finding was bronchopneumonia (48%). Pulmonary infiltrates occurred in six children (20%) under four years of age. Three of the children (10%) had severe clinical and/or radiological manifestations. These findings depict a pattern of mycoplasma infection that is different from previous reports, i.e. high pulmonary infection rates in families, the occurrence of pneumonic infiltrations in young children and a quite severe clinical and radiological course in some of the patients.