A nine month old male child presenting degenerating right ulna (massive osteolysis) has been followed up for two years. The bone completely disappeared due to abscesses on the right forearm and without orthopedic or haematological complications. Repeated lymphocyte cultures showed somatic pairing (mostly chromosome pair 5), end to end association involving chromosome 14, 21, 21 and 16, and satellite enlargement in a high proportion of cells with an otherwise normal 46,XY karyotype. These observations are compared with 13 other types of orthopaedic patients, and we opine that cumulative picture of chromosomal aberrations appears to correspond with the present rare anomaly "Mono Ostolic Osteolysis" involving right ulna. None of the controls or any other orthopaedic anomaly studied hereunder exhibits this chromosomal picture.