Personalized therapies may improve outcomes in elderly patients with brain metastases from cancer of unknown primary (CUP). To contribute to this strategy, an instrument for judging their survival time was designed. This retrospective study included 53 patients, aged ≥65 years and treated with whole-brain irradiation (WBI) for brain metastases from CUP. The WBI-program, age, gender, Karnofsky performance score (KPS), number of brain metastases and non-cerebral metastases were analyzed. KPS≤60 (p<0.001) and presence of non-cerebral metastases (p=0.003) were significantly associated with unfavorable survival. These factors formed the basis for the prognostic implement; patient-scores of zero (n=23), one (n=21) or two points (n=9) were obtained. Corresponding survival rates at 6-months were 0%, 19% and 56% (p<0.001). With this instrument, it is easier to judge the remaining survival time of elderly patients with brain metastases from CUP. This information should be used when selecting individual treatment- and WBI-programs.