Ultrasonographic diagnosis of a large placental intervillous thrombosis associated with elevated maternal serum alpha-fetoprotein level. A placental lesion associated with elevated maternal serum alpha-fetoprotein level and a positive Kleihauer-Betke test is reported. Serial ultrasonograms showed a progressive increase in the echogenicity of the lesion that corresponded to a large intervillous thrombosis after delivery. This case illustrates the importance of detailed ultrasonographic and pathologic placental investigations in pregnancies with unexplained elevated levels of alpha-fetoprotein. 