We assessed prospectively the risk of increased incidence of respiratory symptoms after exposure to particular fungal genera in a susceptible population--namely, infants (n = 880) at high risk for developing asthma. Days of wheeze or persistent cough, information on maternal allergy and asthma, socioeconomic variables, and housing characteristics were collected over the course of the infant's first year of life. Exposure to mold was assessed by airborne samples collected at one time early in the infant's life. Fungi were identified to genus level, recorded as colony-forming units per cubic meter (CFU/m3), and then categorized into four levels: 0 (undetectable), 1-499 CFU/m3 (low), 500-999 CFU/m3 (medium), and greater than or equal to 1,000 CFU/m3 (high). Effects of mold on wheeze and persistent cough, adjusting for potential confounding factors, were examined with Poisson regression analyses. The two most commonly found genera were Cladosporium (in 62% of the homes) and Penicillium (41%). Cladosporium was associated with reported mold (p < 0.02) and water leaks (p < 0.003). Rate of persistent cough was associated with reported mold [Rate ratio (RR) = 1.49; 95% CI, 1.18-1.88]. The highest level of Penicillium was associated with higher rates of wheeze (RR = 2.15; 95% CI, 1.34-3.46) and persistent cough (RR = 2.06; 95% CI, 1.31-3.24) in models controlling for maternal history of asthma and allergy, socioeconomic status, season of mold sample, and certain housing characteristics. We conclude that infants in this high-risk group who are exposed to high levels of Penicillium are at significant risk for wheeze and persistent cough.