Data on incidence, characteristics, and prognosis in stage I childhood anaplastic large cell lymphoma are scarce. Of 463 patients enrolled in the international ALCL99 trial, 36 (8%) had stage I disease and were treated with a prephase chemotherapy, followed by either 3 chemotherapy courses in case of initial complete resection (6 patients) or otherwise by 6 courses of chemotherapy (30 patients). Disease localization was to the peripheral lymph nodes in 26, soft tissue mass in 8, and solitary bone and bronchial disease in 1 patient each. Of the 6 patients with complete resection, none experienced relapse, whereas of the 30 remaining stage I patients, 9 (30%) relapsed, including in all cases a new site of disease involvement and including 3 of 5 anaplastic lymphoma kinase-negative patients. In summary, the failure rate for incompletely resected stage I disease was similar to that for patients with stage II and stage III/IV disease. Whether anaplastic lymphoma kinase negativity contributed to this moderate outcome has to be proven prospectively. This study was registered at www.clinicaltrials.gov as NCT00006455.