BACKGROUND Rapidly involuting congenital hemangioma (RICH) of the fetal skull is an extremely rare vascular disease which undergoes proliferation only in utero and progresses with maximal size at birth. RICH can be detected by prenatal imaging but is easily misdiagnosed. CASE REPORT A 28-year-old nulliparous woman was referred at 38 weeks of gestation for routine screening with obstetric ultrasonography. The ultrasonography revealed a female fetus with a previously undetected head tumor (32×22 mm). Certain unusual sonographic features were observed: the lesion was fusiform, with a wide base adjacent to the frontal bone. Tumor growth appeared to be toward the brain parenchyma rather than outwards (ie, toward the skull), which suggested that the mass may have been derived from the skull. The mass may have remained undiagnosed due to its small size or due to the superimposition of the skull in poor quality ultrasound images. On the basis of ultrasound findings, the lesion was diagnosed as an intracranial tumor, but fetal MRI findings led to the suspicion of RICH of the fetal skull. Finally, the patient was followed up until 1 year after birth, by which time the lesion had completely disappeared. CONCLUSIONS Careful evaluation of prenatal ultrasound is necessary to ensure detection of any mass adjacent to the skull, and the ultrasonography technician should carefully examine the features of any suspected mass to diagnose it correctly to avoid affecting the treatment strategy.