OBJECTIVES: The evaluation of hypomagnesemia's significance in predicting the presence of the black hole sign in patients with intracranial hemorrhage is currently under investigation. METHODS: The study included 261 patients with cerebral hemorrhage who underwent initial skull computed tomography within 24 hours of admission. Sixty-nine patients (26.4%) exhibited hypomagnesemia in the initial laboratory examinations. The black hole sign was observed in 123 patients (referred to as the black hole sign group, which includes patients with and without hypomagnesemia), while the remaining 138 patients (nonblack hole sign group) did not exhibit this feature. The values of hypomagnesemia were assessed through multivariable logistic regression analyses. RESULTS: The black hole sign occurred in 45 of the 69 (65.2%) patients with hypomagnesemia, and in 78 of the 192 (40.6%) patients without hypomagnesemia. In the black hole sign group, hypomagnesemia was observed in 45 patients (36.6%). However, only 24 patients (19.5%) from the normal magnesium concentration group exhibited hypomagnesemia. The sensitivity, specificity, and positive and negative predictive values of hypomagnesemia for predicting the black hole sign were 69.9%, 82.5%, 36.6%, and 82.8%, respectively. The odds ratios for hypomagnesemia, smoking history, and hypokalemia in predicting the presence of the black hole sign were 2.74, 1.971, and 1.629, correspondingly. CONCLUSIONS: The presence of hypomagnesemia may serve as a predictive factor for the black hole sign and rebleeding in patients with intracerebral hemorrhage, thereby providing valuable guidance for clinical treatment.