The second-to-fourth digit ratio (2D:4D) has been associated with sexual dimorphism, with a lower 2D:4D in males. A large body of research has relied on the 2D:4D as a proxy for prenatal androgen exposure, and includes reports of relationships between 2D:4D and a wide range of human traits. Here, we examine the validity of the 2D:4D proxy by studying the association between 2D:4D and classical Congenital Adrenal Hyperplasia (CAH) due to 21-hydroxylase deficiency, a condition characterized by excessive prenatal exposure to androgens during most of the gestational period. To this end, we retrospectively examine serial radiographs of the left hand obtained clinically in 91 youth with classical CAH (49.5% female) and 70 control youth (44.3% female). Replicating previous reports, we observe associations of the 2D:4D with sex (lower 2D:4D in males), age (increase of 2D:4D through development) and ethnicity (lower 2D:4D in Hispanic individuals). However, we find no evidence for differences in 2D:4D between CAH and controls (full sample: □ = 0.0003 [-0.007, 0.008]; females: □ = −0.004 [-0.015, 0.007]; males: □ = 0.003, [-0.007, 0.013]). Although our findings do not rule out a small association between the 2D:4D and CAH, they cast doubt on the usefulness of the 2D:4D as a biomarker for prenatal androgen exposure in behavioral research.