Degree and extent of lytic bone lesions were quantified at diagnosis in 69 patients with myelomatosis (MM). longest survival was found in patients with minimal lytic lesions and shortest survival was found in those either without bone lesions or with severe and extensive lesions. Among the 12 patients without bone lesions only 6 were Durie & Salmon stage I and only 1 later developed a single lesion. Based on these data, bone involvement in MM does not seem to be a reliable predictive factor of survival in all cases. This suggests the existence of 2 distinct types of MM: 1 with tumour and 1 without. Lytic lesions could have a prognostic value only in the 1st type.