Δ9-Tetrahydrocannabinol (THC) treatment inhibits the xenograft growth of lung tumors in SCID mice. A549 cells (3 × 106) were injected subcutaneously into the right flank of immunodeficient SCID mice. THC was given at a dose of 5 mg/kg/day peritumorally for 21 days (n=6 for each experimental group) after palpable tumors were observed. Tumor size was measured every week. After 3 weeks, significant decreases in tumor volume (a) and tumor weight (b) were observed in the THC-treated animals compared to the vehicle-treated animals (**P<0.001; compared to vehicle-treated animals).