Outpatient adherence to oral medication regimens was evaluated in 50 adolescents and young adults with cancer: 21 patients with acute lymphoblastic leukemia or Hodgkin disease who were to take prednisone as a component of their chemotherapy regimen, and 29 patients with Hodgkin disease whose chemotherapy had been discontinued and who were to take penicillin for postsplenectomy prophylaxis. Of the 21 adolescent patients with acute lymphoblastic leukemia or Hodgkin disease who were to take prednisone as outpatients, 11 patients were found to be nonadherent to treatment (52%). Among the 29 adolescents for whom penicillin was prescribed for postsplenectomy prophylaxis, nonadherence was detected in 14 patients (48%). If a method is available, laboratory assessment to determine adherence should be performed in adolescents and young adults receiving long-term therapy, particularly if one wishes to draw valid conclusions regarding efficacy of various treatment protocols.