A sacrococcygeal adenocarcinoma is reported in a male patient nearly 40 years old following resection of a histologically mature sacrococcygeal teratoma. The adenocarcinoma arose within soft tissue of the presacral area and residual coccyx and extensively invaded the coccygeal stump that had not been removed in toto with the teratoma during initial surgery at 2 months of age. The patient died nearly 2 years later with local and regional recurrence of tumor and metastases to lymph nodes, liver, lung, bone, and brain. At autopsy there was no evidence of origin from deep internal organs such as the stomach, pancreas, or other sites. Brief comments are made about malignant tumors complicating sacrococcygeal teratomas in both the pediatric age group and adults, and attention is focused on other lesions entering into the differential diagnosis, particularly retrorectal cyst/hamartoma or tailgut cysts. This case underscores an extremely rare but potentially fatal outcome of a patient with sacrococcygeal teratoma.