Abstract Context Testicular adrenal rest tumors (TART) are a common complication in males with classic 21-hydroxylase deficiency (21OHD). TART are likely to contribute to the androgen excess in 21OHD patients, but a direct quantification of steroidogenesis from these tumors has not been yet done. Objective We aimed to define the production of 11-oxygenated 19-carbon (11oxC19) steroids by TART. Methods Using liquid chromatography-tandem mass spectrometry, steroids were measured in left (n = 7) and right (n = 4) spermatic vein and simultaneously drawn peripheral blood (n = 7) samples from 7 men with 21OHD and TART. For comparison, we also measured the peripheral steroid concentrations in 5 adrenalectomized patients and 12 age- and BMI-matched controls. Additionally, steroids were quantified in TART cell– and adrenal cell–conditioned medium, with and without adrenocorticotropic hormone (ACTH) stimulation. Results Compared with peripheral blood from 21OHD patients with TART, the spermatic vein samples displayed the highest gradient for 11β-hydroxytestosterone (11OHT; 96-fold) of the 11oxC19 steroids, followed by 11-ketotestosterone (47-fold) and 11β-hydroxyandrostenedione (11OHA4; 29-fold), suggesting production of these steroids in TART. TART cells produced higher levels of testosterone and lower levels of A4 and 11OHA4 after ACTH stimulation compared with adrenal cells, indicating ACTH-induced production of testosterone in TART. Conclusion In patients with 21OHD, TART produce 11oxC19 steroids, but in different proportions than the adrenals. The very high ratio of 11OHT in spermatic vs peripheral vein blood suggests the 11-hydroxylation of testosterone by TART, and the in vitro results indicate that this metabolism is ACTH-sensitive.