We sought to measure the anxiety felt by parents at the time of entry into a randomized controlled vaccine trial, and to determine if anxiety level was associated with parental demographic variables or past experience. The children were 2-month-old infants entering a randomized controlled clinical trial (RCT) of a diphtheria-tetanus toxoid-acellular pertussis vaccine adsorbed with Haemophilus influenzae B conjugate, or toddlers enrolling in a RCT of a Meningococcal C conjugate vaccine. Nurses interviewed parents to collect demographic data and parents self-administered the Spielberger Self-evaluation Questionnaire (State Anxiety STAI-Y-I) [Manual for the State-Trait Anxiety Inventory (Form Y) (Self-evaluation Questionnaire), Consulting Psychologists Press Inc., Palo Alto, 1983], a validated instrument measuring the temporary condition of "state anxiety." A regression tree (CART) (S-Plus) was used to identify factors associated with higher anxiety scores. Parents of 97 children enrolled. Anxiety scores ranged from 22.75 (lower anxiety) to 36.43 (higher anxiety). The regression tree identified a structured tree with six branches. The highest anxiety scores occurred in fathers with education less then grade 8, mothers with education less than high school, birth order of the child less then the third, previous serious illness in the family, or lack of experience with research. In a group of parents agreeing to enroll their infant or toddler in a vaccine study, certain attributes and experiences were associated with higher anxiety at the time of immunization in the context of a RCT. These factors should be considered by vaccine researchers in the recruitment process of clinical trials.