This paper presents indoor air pollutant concentrations and allergen levels collected from the homes of 100 Baltimore city asthmatic children participating in an asthma intervention trial. Particulate matter (PM), NO2, and O3 samples were collected over 72 h in the child's sleeping room. Time-resolved PM was also assessed using a portable direct-reading nephelometer. Dust allergen samples were collected from the child's bedroom, the family room, and the kitchen. The mean PM10 concentration, 56.5+/-40.7 microg/m3, is 25% higher than the PM2.5 concentration (N=90), 45.1+/-37.5 microg/m3. PM concentrations measured using a nephelometer are consistent and highly correlated with gravimetric estimates. Smoking households' average PM2.5 and PM10 concentrations are 33-54 microg/m3 greater than those of nonsmoking houses, with each cigarette smoked adding 1.0 microm/m3 to indoor PM2.5 and PM10 concentrations. Large percentages of NO2 and O3 samples, 25% and 75%, respectively, were below the limit of detection. The mean NO2 indoor concentration is 31.6+/-40.2 ppb, while the mean indoor O3 concentration in the ozone season was 3.3+/-7.7 ppb. The levels of allergens are similar to those found in other inner cities. Results presented in this paper indicate that asthmatic children in Baltimore are exposed to elevated allergens and indoor air pollutants. Understanding this combined insult may help to explain the differential asthma burden between inner-city and non-inner-city children.