The PREVASC study addresses the primary prevention of asthma in infants and small children. The objective of this study is to investigate whether a multifaceted prenatally started intervention strategy in high-risk infants leads to a decrease in the occurrence of (severe) asthma and whether a refinement of the prevention strategy leads to an increase in the adherence to the prevention program. The primary prevention program includes house dust mite impermeable bed coverings, education on breast feeding, hypoallergenic feeding, timing of introduction of solid food and smoking cessation. A total of 888 infants were prenatally included. By the time of inclusion the mothers were 3-7 months pregnant. About 27 infants were excluded from the study and 18 dropped out. Of the remaining 843 infants 535 had a first-degree familial predisposition of asthma (high-risk group), whereas a reference group of 308 (162 boys) infants was not predisposed for asthma in the first-degree (low-risk group). To evaluate the (cost-)effectiveness of the preventive intervention, 222 (118 boys) infants of the high-risk group allocated to the intervention group and 221 (112 boys) allocated to a control group are followed up. The low-risk infants served as controls to evaluate the predictive value of high risk (first-degree familial predisposition of asthma). The infants are followed from the prenatal stage until they reach the age of 6 yr. The remaining 92 high-risk infants were included in an optimized randomized-clinical adherence trial (RCAT). Of these 92 infants, 45 (20 boys) were allocated to an intervention group and 47 (24 boys) to a control group. Until now all infants have been followed for at least 1 yr.