A breakthrough in understanding of mechanisms of bone structure regulation has brought about the introduction of the new synthetic recombinant human parathyroid hormone 1-34 (PTH1-34; Teriparatide) in the treatment of osteoporosis. These mechanisms, involving the RANKL, RANK, and osteoprotegerin system, are also known to be involved in malignant myeloma (MM) and tumor and bone metastasis development. We report a case in which MM was found after treatment of osteoporosis with teriparatide. We were unable to demonstrate any direct association between the MM and teriparatide treatment. However, it seemed intriguing that similar mechanisms are activated in the development of MM as those being working during teriparatide treatment. In the view of our case, we propose that MM by examination of serum protein fraction should be searched for prior to treatment with teriparatide as it is an exclusion criterion in teriparatide treatment of secondary osteoporosis. A search for other metastatic diseases prior to teriparatide treatment should eventually also be considered. The theoretical basis for our proposal is discussed.