Background: To verify the importance of Blomia tropicalis in atopic dermatitis (AD), we determined the cutaneous reactivity and the serum level of B. tropicalis‐specific IgE and IgG subclasses in AD patients. Methods: B. tropicalis‐specific IgE and IgG subclasses were determined in AD patients and compared with bronchial asthma (BA) patients and a control group (CG) of nonatopic subjects. Specific IgE was obtained by skin prick test and RAST. B. tropicalis‐specific IgG subclasses were determined by ELISA. The data were statistically analyzed by chi‐square test (Mantel‐Haenszel) and odds ratio (OR). Results: We detected positive skin prick tests in 61.76% of AD and 83.33% of BA patients, and in 12.5% of the CG. RAST was positive in 44.12% of AD and in 61.90% of BA patients, but not in the CG. B. tropicalis‐specific IgG1 and IgG2 subclasses showed no significant differences between the three groups. IgG3 subclass positivity was statistically significant in AD patients (41.17%) when compared to BA patients (14.29%) and the CG (16.67%). The determination of B. tropicalis‐specific IgG4 was positive in 32.35% of AD patients, 21.43% of BA patients, and 8.33% of the CG. Conclusions: These results confirm that the storage mite B. tropicalis is an important allergen in AD. It is possible that IgG3 activates the complement in AD patients, releasing vasoactive amines that further amplify the allergic reaction. The positive results of the B. tropicalis‐specific IgG4 found in AD and BA were probably due to chronic exposure to this storage mite in the home environment.