Many patients with eczematous dermatitis need continued care in case of a recurrent or persistent skin condition and potential adverse effect of medications. Allergic contact dermatitis should be considered in the differential diagnosis of eczematous dermatitis especially in a patient whose dermatitis is persistent despite appropriate therapies. Patch testing is an essential investigation in patients with persistent eczematous eruption when contact allergy cannot be ruled out. The purpose of this study was to determine the frequency of contact allergy in patients with eczematous dermatitis in Thammasat University Hospital, Prathumthani, Thailand from June 1, 2008 to June 30, 2009 and to identify a possible relationship between sex, age, occupational differences and type of eczema that is associated with positive patch test reactions. A total of 157 patients were patch tested with 23 standard allergens. One or more positive responses were noted in 70 patients (44.6%). The most common allergen was nickel sulfate (26.8%), followed by cobalt chloride (7.6%), p-phenylenediamine (7.0%), fragrance mix (7.0%). Patients who were initially diagnosed with allergic contact dermatitis had significant correlation with positive patch test results to nickel sulfate, cobalt chloride and phenylenediamine (p = 0.00, p = 0.03, p = 0.02, respectively). Patients who were initially diagnosed with endogenous eczema had significant correlation with positive patch test results to colophony (p = 0.04). Contact allergy to fragrance mix was significantly more frequent in patients who had personal history of atopy (p = 0.04). There was no significant correlation between the frequency of contact allergy and sex, age, location of lesion and patient’s occupation. In conclusion, this study demonstrated the prevalence in contact allergy in eczema patients in Thammasat University Hospital and compared the results with other region from Thailand. Further study involving many hospitals in various areas in Thailand is needed to provide more insight into contact allergy in Thailand.