Anaplastic lymphoma kinase (ALK) is constitutively activated in a number of human cancer types due to chromosomal translocations, point mutations, and gene amplification and has emerged as an excellent molecular target for cancer therapy. Here we report the identification and preclinical characterization of CEP-28122, a highly potent and selective orally active ALK inhibitor. CEP-28122 is a potent inhibitor of recombinant ALK activity and cellular ALK tyrosine phosphorylation. It induced concentration-dependent growth inhibition/cytotoxicity of ALK-positive anaplastic large-cell lymphoma (ALCL), non-small cell lung cancer (NSCLC), and neuroblastoma cells, and displayed dose-dependent inhibition of ALK tyrosine phosphorylation in tumor xenografts in mice, with substantial target inhibition (>90%) for more than 12 hours following single oral dosing at 30 mg/kg. Dose-dependent antitumor activity was observed in ALK-positive ALCL, NSCLC, and neuroblastoma tumor xenografts in mice administered CEP-28122 orally, with complete/near complete tumor regressions observed following treatment at doses of 30 mg/kg twice daily or higher. Treatment of mice bearing Sup-M2 tumor xenografts for 4 weeks and primary human ALCL tumor grafts for 2 weeks at 55 or 100 mg/kg twice daily led to sustained tumor regression in all mice, with no tumor reemergence for more than 60 days postcessation of treatment. Conversely, CEP-28122 displayed marginal antitumor activity against ALK-negative human tumor xenografts under the same dosing regimens. Administration of CEP-28122 was well tolerated in mice and rats. In summary, CEP-28122 is a highly potent and selective orally active ALK inhibitor with a favorable pharmaceutical and pharmacokinetic profile and robust and selective pharmacologic efficacy against ALK-positive human cancer cells and tumor xenograft models in mice.