MYC overexpression is thought to induce tumorigenesis through a variety of different mechanisms including the induction of proliferation, inhibition of differentiation and disruption of genomic stability. Conventional transgenic systems that have been used to study the role of oncogenes in tumorigenesis continuously overexpress transgenes and hence preclude the investigation of the initial and age specific consequences of oncogene activation. To investigate the developmental specific consequences of MYC overexpression in the pathogenesis of lymphoma in vivo, we used transgenic mice in which the MYC proto-oncogene is conditionally regulated via the Tetracycline Regulatory System (Tet system). The ability of MYC to induce lymphomagenesis was found to be inversely correlated with the age of the host at the time of MYC activation. When MYC was activated constitutively, the mean time until tumor development was 13 weeks. When MYC was activated at increasing developmental ages of 3 and 5 weeks, identical tumors arose, but with an increased mean tumor latency. When MYC was activated at 7 weeks, an age equivalent to an adult mouse, no tumors developed even after 40 weeks of observation. However, we found that in adult mice, if both MYC and BCL2 are overexpressed at 7 weeks of age, mice succumbed to lymphoma with a mean latency of 27 weeks. Surprisingly, we could not find evidence that MYC overexpression induces apoptosis or that BCL2 overexpression reduced apoptosis. We conclude that the ability of MYC to induce lymphomagenesis is highly dependent on the developmental context. MYC and BCL2 cooperate to permit tumorigenesis in adult mice. Since lymphoma occurs generally as a disease in adults, this suggests that previous reports may have greatly overestimated the ability of MYC activation to induce tumorigenesis and underestimated the potential cooperation between MYC and BCL2 oncogenes. Our results also suggest that BCL2 may cooperate with MYC to induce lymphomagenesis through additional mechanisms other than preventing MYC from inducing apoptosis.