Because children diagnosed with WNT-activated medulloblastoma have a 10-year overall survival rate of 95%, active long-term follow-up is critically important in reducing mortality from other causes. Here, we describe an 11-year-old adopted female who developed multiple pilomatrixomas 3 years after diagnosis of WNT-activated medulloblastoma, an unusual finding that prompted deeper clinical investigation. A heterozygous germline APC gene mutation was discovered, consistent with familial adenomatous polyposis. Screening endoscopy revealed numerous precancerous polyps that were excised. This case highlights the importance of long-term follow-up of pediatric cancer survivors, including attention to unexpected symptoms, which might unveil an underlying cancer predisposition syndrome.