The etiology, diagnosis and elucidation of non-manifest late infection following total hip joint replacement surgery are discussed on the basis of 22 patients who underwent further surgery in our clinic due to an infected prosthesis from 1974 to 1976. Non-manifest late infection is usually the result of contamination of the hip before, during or after surgery; only in a few isolated cases can the infection be considered to have originated from the bloodstream. There is a higher risk of infection in patients who have undergone previous hip surgery. The principal symptom of late infection is pain; as a rule there are no other clinical signs of inflammation. The generally increased blood sedimentation rate is the only laboratory test of diagnostic value. An important aid is x-ray of the hip, in which the infection is manifested by osteitis and loosening of the prosthesis. Arthrocentesis should be carried out in case of doubt and when systemic antibiotic treatment is envisaged. Special diagnostic procedures such as arthrography and scintigraphy cannot be recommended for routine examination.