Mother, father (26 y.o.) and their only child (5 y.o.) developed nonproductive cough, fever (39.5 to 40.4 degrees C) and bilateral pulmonary infiltrates within three weeks. In addition the mother developed a small left pleural effusion and a pericardial effusion, a relative bradycardia, a pruritic vesicular exanthem of the extremities and the trunk, an erythema nodosum and arthritis of the tarsal joints. The father's coulter counter red blood count was distorted by microagglutination at room temperature (hemoglobin 13.2 gr/dl; erythrocytes 1,91 X 10(6) mm-3 and MCH 69.1 pg; MCV 120 fl and hematocrit 23.8%) but not at 37 degrees C (13.2; 4.15 and 31.8; 92 and 39.3, respectively). In the daughter myringitis, pharyngitis, cervical lymphadenopathy and splenomegaly were observed. Cold agglutinins and serologic evidence for mycoplasma pneumoniae infection were demonstrable in all three. Treatment with Tetracycline (parents) and Erythromycin (child) was effective.