The birthdate-based biorhythm (BBB) hypothesis was examined for utility as a predictor of human performance. Data from quizzes of 26 students taken periodically throughout a semester, and measures over 1 month of landing performance by seven pilots were analyzed by multiple regression/correlation methods. Regression equations were developed to test the correspondence between performance and cycle phases. A second analysis used a nonorthogonal least-square spectrum method to determine if the data contained any systemic rhythms in the intradian range. No significant results were obtained whcih would support the BBB hypothesis as a predictor of human performance. Also, no evidence was found to substantiate the existence of the three proposed BBB cycles.