We examined the psychometric characteristics of the Occupational Self Assessment (OSA), which measures clients’ perceptions of their own competence and the value they assign to occupations. Two hundred ninety-six adolescents with acute mononucleosis completed the OSA, the Fatigue Scale, the Checklist of Infectious Symptoms, the Child Health Questionnaire, and the Perceived Stress Scale. OSA items coalesced to capture the intended constructs; the rating scales functioned as intended. More than 90% of adolescents were validly measured. The OSA showed adequate sensitivity and was stable over time. OSA measure of competence was moderately associated with infectious symptoms, fatigue severity, health status, and stress, and the measure of values was not. Neither measure was associated with age, gender, or ethnicity. Finally, adolescents who had not recovered from mononucleosis after 12 mo showed lower competence scores yet attached the same value or importance to occupational participation as adolescents who had recovered.