The p16 gene (MTS1, CDKN2, p16INK4A, CDKI) encoding an inhibitor of cyclin-dependent kinase 4 (cdk4) has been found to be deleted in various types of tumors, including leukemia, and is thought to code for a tumor suppressor gene. Our preliminary findings on eight pediatric patients with acute lymphoblastic leukemia (ALL) suggested that the survival of patients carrying a homozygous p16 gene deletion was significantly inferior to that of those without a deletion. The present study on 48 patients tested the hypothesis that the clinical outcome for pediatric ALL patients is correlated with the presence or absence of the p16 gene. Overall, nine of 48 children (18.3%) carried a homozygous p16 deletion. Such deletions were significantly more common (P = .003) among T-ALL patients (five of eight, 62.5%) than among precursor-B-ALL patients (four of 40, 10.0%). Of nine patients exhibiting p16 deletions, eight (88.9%) were classified as high-risk patients by the recognized prognostic factors of age, white blood cell count, and T-cell phenotype. The 4-year event-free survival in the study population as a whole was 72.7%. Without adjustment for other risk factors (univariate model), the presence of a homozygous p16 deletion was associated with a markedly increased probability of both relapse (P = .0003) and death (P = .002). These findings raise the question of whether the p16 deletion itself confers an increased risk of relapse after adjusting for the known risk factors. In this analysis, the estimated risk multiplier factor for relapse in patients carrying the p16 deletion was 14.0 (P = .0004) and for the risk of death 15.6 (P = .0008). We therefore conclude that the presence of a homozygous p16 deletion may well be an important risk factor for both relapse and death in childhood ALL, and that its prognostic effect is not a consequence of confounding by other factors already known to influence outcome in this disease.