The aim of this study was to investigate the prevalence of house dust mite sensitization and to detect the IgE and eosinophilic cationic protein (ECP) levels in the positive cases living in Kocaeli province (located in Northwest region of Turkey). The results of mixed house dust mite skin tests of 1279 patients (620 male, 659 female) of which 670 were children (3-14 years), 68 were adolescents (15-19 years) and 541 were adults (20-86 years) who were admitted to Kocaeli University Medical Faculty Hospital during the period of January 2003-January 2005, with symptoms of atopic disease such as upper/lower respiratory tract symptoms, asthma and/or rhinitis and/or eczema were evaluated retrospectively. Serum IgE and ECP levels of all cases were determined by enzyme immunoassay method (UniCAP 100 System, Pharmacia, Uppsala, Sweden). The allergy test performed by using mixed house dust mite panel (Dermatophagoides pteronyssinus, Dermatophagoides farinae) was found positive in 328 (25%) cases. Among those 154 (46.9%) were sensitive to D.pteronyssinus, 151 (46%) were sensitive to D.farinae, and 23 (7%) were sensitized to both mites. The mean IgE (259 +/- 25.5 kU/ml) and ECP levels (35.7 +/- 17.9 kU/ml) of house dust mite sensitized cases were found significantly higher than the non-sensitized subjects (IgE; 144 +/- 18.9 kU/ml, ECP; 33.1 +/- 18.3 kU/ml) (p< 0.001). The prevalence rates of house dust mite allergy in our region were determined as 17% (114/670), 19.1% (13/68), and 37.1% (201/541) in pediatric, adolescent and adult populations, respectively. There was no predilection for allergy prevalence in terms of sex (p= 0.7). However, the prevalence increases during autumn-winter seasons (p= 0.01 and p= 0.007, respectively). In conclusion, it was determined that the rate of the sensitization against house dust mite was high in Kocaeli, Turkey and it exhibited an increasing tendency with age.