BACKGROUND Contact allergy to cosmetic ingredients is common. However, there are no recent comprehensive studies on contact allergy to cosmetic ingredients in Asia. OBJECTIVE To identify positive patch test reactions in patients tested at Siriraj Hospital, Bangkok, Thailand to allergens present in cosmetics. METHODS A retrospective review of medical records from the outpatient contact dermatitis clinic was conducted from January 1999 to December 2008. Patients with at least one positive patch test reaction to allergens associated with cosmetic ingredients were studied. The results were evaluated using Pearson's χ(2) -test with Yates' continuity correction or Fisher's exact test where appropriate,and a p-value <0.002 was considered to be statistically significant by Bonferroni correction. RESULTS There were 1247 cases (239 males and 1008 females; mean age 38.5 years). Fragrance chemicals and preservatives were the most commonly recognized cosmetic allergens. Ammoniated mercury was the only allergen that showed a significantly increased frequency over the 10-year period (p = 0.0008). CONCLUSIONS Our study showed that ammoniated mercury is an emerging cosmetic allergen, showing an increased prevalence in recent years in Thailand. A focus is required on emerging cosmetic allergens and what may account for the upward trend of cosmetic contact dermatitis.