Three years after diagnosis of chronic lymphocytic leukaemia in a 57 year old man developed a hypercalcaemia with multiple bone fractures, concomitant an increase of activity of the leukaemia. There was no hyperparathyroidism, nor in serum, nor inside the lymphocytes. The osteopenia was caused by leukaemic infiltrations. An additional activation of osteoclasts was caused by an osteoclasts activating factor (OAF), produced by the leukaemic cells. 2 (= 2.25%) of 89 Non-Hodgkin-Lymphomas except chronic lymphatic leukaemia had a moderate hypercalcaemia with concomitant activation of the underlying disease. 9 (= 7.7%) of 116 chronic lymphocytic leukaemias had hypercalcaemia, 2 thereof with increased activity of the leukaemia. Hypercalcaemia is thus very rarely found in other Non-Hodgkin-Lymphomas, in chronic lymphocytic leukaemia some what more often, and only here a concomitant increase in the activity of the underlying disease could be observed in some cases.