Germ-line DICER1 mutations predispose to a distinctive tumour predisposition syndrome, the DICER1 syndrome, which is associated with a spectrum of rare mainly childhood-onset tumours. In 2014, a case of well-differentiated fetal adenocarcinoma of the lung (WDFA) was reported in a 16-year-old germ-line DICER1 mutation carrier. Here we report our finding of a characteristic somatic DICER1 RNase IIIb c.5127T>A (p.Asp1709Glu) missense mutation within the WDFA, confirmed using laser capture microscopy. The child has a personal history consistent with the DICER1 syndrome: she developed a multinodular goitre at age 14 years and an ovarian Sertoli-Leydig cell tumour at age 16 years, each of which were found to harbour a somatic DICER1 RNase IIIb missense mutation. The identification of two DICER1 “hits” in the WDFA strongly suggests that WDFA is a rare, previously-unrecognised manifestation of DICER1 syndrome.