Background: Carba mix (CM) contains 3 chemicals used as accelerators in manufacturing of rubber products and agricultural chemicals. Iodopropynyl butylcarbamate (IPBC) is a preservative used in industrial and personal care products. Potential cross-reactivity between these allergens is unclear. Objective: This study aimed to determine concomitant reaction rates between CM (3% petrolatum [pet]) and IPBC (0.1% pet and/or 0.5% pet). Design: A retrospective cross-sectional analysis of data from the North American Contact Dermatitis Group from 1998 to 2008 was conducted. Frequencies of positive reactions, strength of reactions, and concomitant reaction rates were calculated. Results: A total of 25,435 patients were tested. There were 1131 allergic reactions to CM and 346 positive reactions to IPBC (either 0.1% pet and/or 0.5% pet). Fifty-two patients reacted to both allergens. Most reactions (969%) were doubtful/weakly positive, and of those who reacted to both, most (58%) had doubtful and/or weakly positive reactions. There was a statistically significant association of concomitant reactions between CM and IPBC; formaldehyde positivity was used as control and showed statistically significant concomitant reactions. Conclusions: Overall, concomitant reactions to CM and IPBC were low, and rates varied by strength of reaction. True cross-reactivity is unlikely; statistical association is likely due to frequent low-grade reactions to irritant patch test preparations.