BACKGROUND/OBJECTIVE: This study characterizes concomitant reactions to carba mix (CM) and thiuram mix (TM) in a large North American population. Because thiurams and dithiocarbamates have structural similarity, concomitant reactions are expected. METHODS: The 1994-2016 North American Contact Dermatitis Group data were analyzed. Patients with a final reaction interpreted as "allergic" to either CM or TM were included. RESULTS: A total of 49,758 patients were tested to both CM and TM. A total of 3437 (6.9%) had positive reactions to CM and/or TM including the following groups: CM+ only (n = 1403, 40.8%), TM+ only (n = 1068, 31.0%), or both (n = 966, 28.1%). A total of 47.5% of TM+ patients were positive to CM and 40.8% of CM+ patients were positive to TM. Male sex, occupationally related dermatitis, and hand involvement were significantly more common in individuals positive to CM and/or TM as compared with those who were negative (P < 0.0001). More than 80% of CM+/TM+ reactions were currently relevant. Gloves were the most common source of CM and TM; clothing and footwear were also frequent. CONCLUSIONS: Carba mix and TM remain important, clinically relevant allergens. Although significant concomitant reaction frequency was demonstrated, more than half of the patients reacting to either CM or TM would have been missed if both had not been tested, underscoring the importance of testing to both.