Grip strength measurements have proven useful in the assessment of children's physical growth and development. Mathiowetz, Weimer and Federman (1986) found that gains in grip strength coincided with chronological age and that males were stronger than females at all ages. They also noted that hand dominance did not significantly influence hand strength scores. The purpose of this research was to examine normative changes in grip strength performance by males and females with emphasis on age-sex interactions. The participants in this study were 736 healthy children and youth ages 6 to 19 enrolled in rural and urban public schools in the northeastern United States. Each participant was tested with a calibrated, hydraulic dynamometer (manufactured by Country Technology Inc., Model # 68840). Procedures recommended by the American Society of Hand Therapists were followed throughout testing. Left grip (lmean), right grip (rmean), and dominant hand grip strength (DHGS) each served as the dependent variable in three separate analyses. To test for an age/sex interaction, the dependent variable was regressed on age (centered at the mean), sex, and the product of age and sex. A statistically significant interaction surfaced for each dependent variable. That is, the relationship between age and grip strength was different for males and females or, stated equivalently, the magnitude of the sex difference in grip strength varied with age. For example, DHGS for females increased 4.4 pounds with each additional year, whereas the corresponding figure for males was 8.6 pounds. Interpreted equivalently, there was a 17.6 pound DHGS difference favoring males when assessed at the mean age (12.8 years), but this sex difference increased 4.2 pounds with each additional year. The DHGS sex difference was negligible up through age 12, at which point the male and female means grew increasingly disparate. The interaction of age and sex explained an additional 6% of the variance in DHGS beyond the 68% explained by age and sex separately. Results were similar for the other two dependent variables, lmean and rmean. . The current research confirms previously reported findings of biological differences between males and females; however, the examination of the age and sex interaction provides an additionally useful way of looking at data if there is interest in valid and reliable measures of behavior change over time.