Clinical observations of 11 cases of acute pyogenic osteomyelitls and 14 cases of acute pyogenic arthritis in infancy during past 11 years were carried out and following results obtained. 1. The highest age incidence was less than 3 months after birth (70.6%). 2. Average duration of symptoms was 7.8 days. 3. The most important underlying cause was multiple femorai vessel puncture (23.5%). 4. The chief symptom or sign on admission was pseudoparalysis or limitation of motion (82.4%), and local swelling (82.4%). But high fever on admission was present in only 29.4% of cases. 5. The femur was the most common site in acute pyogenic osteomyelitis (36.4%), and the hip joint in acute pyogenic arthritis (42.9%). 6. Leucocyte count was Increased more than 20,000/mm in 60.0% of the cases. Erythrocyte sedimentation rate was increased more than 21 mm/hr in 72.0% of cases. The positive result on bacteriologic culture was 76.0%. Coagulase-positive Staphylococcus aureus was cultured in 4 cases of acute pyogenic osteomyelitis and 5 cases of acute pyogenic arthritls. a-hemolytic Streptococcus was cultured in 2 cases of acute pyogenic osteomyelitls and 1 case of acute pyogenic arthritis. Hemophilus influenzae was cultured in 1 case of acute pyogenic osteomyelitls and 2 cases of acute pyogenic arthritis. 7. 45.5% of acute pyogenic osteomyelitis was combined with acute pyogenic arthritis. 8. The longer the duration of symptoms was, the longer the abnormal hematologic picture lasted. Erythrocyte sedimentation rate was the most accurate index of activity in acute infection of bone and joint. 9. Shortening or deformity due to epiphyseal plate damage occured in 1 case of acute pyogenlc osteo-myelitis, and 3 cases of acute pyogenic arthritis. Those complications could be minimized wlth early diagnosis and treatment.