Endotoxins (ET) are pro‐innammatory substances present in hou.sc dust which may increase non‐specific bronchial reactivity in asthmatic patients. Endotoxins (EU/g) and Der p I levels were compared in the homes often asthmatic and ten control children, aged 6‐16 years, living in Sao Paulo, Brazil. The houses were visited once a month from February 1993 lo February 1994 and dust samples were collected from the bedding and floor of each subject's house. No significant differences were observed in ET and Derp 1 levels in the homes of asthmatics and controls. The highest ET levels were detected in January and November, whereas the lowest levels were detected in April and August (p < 0.05). demonstrating a distinct seasonal distribution. The highest Der p I levels in bedding were observed in July and the lowest in February (p < 0.05). while Derp I levels in floor did not show significant differences throughout the year. Symptom and medication scores were evaluated monthly in the group of asthmatic children. There was a significant correlation (p < 0.05. r = 0.63) between clinical symptom scores and ET exposure, however no significant correlation was found for mite exposure (p > 0.05. r = 0.19). The results suggest that ET exposure exacerbates asthmatic symptoms in mite allergic, asthmatic children.