Hypercalcemia in monoclonal gammopathy of undetermined significance (MGUS) presents a clinical challenge since it may indicate progression to multiple myeloma (MM) but could also be due to a multitude of unrelated disorders. To inform the approach to this clinical challenge, we conducted a nested cohort study within the iStopMM screening study. Of the 75,422 Icelanders aged 40 years and above who underwent screening for MGUS, we included 2,546 with MGUS who were in active follow-up, …