The results of a program of research designed to produce an adequate measure of the type A behavior pattern in children are reported. The type A pattern is a risk factor for heart disease in adulthood and is characterized by extremes of competitiveness, impatience, easily aroused anger, and aggression. A questionnaire called the Matthews Youth Test for Health (MYTH-Form O) contains 17 statements that characterize pattern A behaviors in children. Study 1 was conducted to provide psychometric data for the MYTH. Teachers rated how well these statements characterized 485 children enrolled in grades K, 2, 4 and 6. Those children remaining in the school district 3 months later (N = 420) were rated again. Statistical analyses of these ratings suggest that the MYTH-Form O is a reliable, internally consistent instrument, which yields 2 orthogonal factors: competitiveness and impatience-aggression. As expected, there were substantial gender differences in children's type A behavior. Study 2 tested the construct validity of the MYTH in a subsample of children who were challenged to win a car race against an experimenter; were given an opportunity to play with a variety of toys, including a plastic Bobo doll; and were asked to execute a frustrating task during 1 session. Results showed that type A's won a race against a female (not a male) experimenter by a larger margin than did type B's. Type A's aggressed against a Bobo doll earlier and were more impatient than were type B's throughout the session. These impatient behaviors exhibited by child type A's are similar to those exhibited by adult type A's during the standardized adult type A interview. In sum, these data are supportive of the reliability and validity of the MYTH and represent a first step in the development of an instrument to assess pattern A in elementary school-aged children.