 Zoledronic Acid Zometa, a new generation, highly potent bisphosphonate used to treat osteoporosis and hypercalcemia of malignancy, is widely used in adult malignancies with potential for bone metastasis such as breast cancer, multiple myeloma and prostate cancer. Bisphosphonates modulate the bone environment by toxicity to osteoclasts resulting in decreased bone resorption. Zometa is the first bisphosphonate to affect both osteolytic and osteoblastic metastatic lesions. In several large randomized studies in adults with recurrent or advanced malignancies, patients randomized to Zometa had delay in progression of bone metastases and less morbidity skeletal related events when compared to either placebo or pamidronate. The toxicity profile of Zometa in adults has been tolerable and includes hypocalcemia, temperature rise, and nausea. The most concerning toxicity is decline in renal function that appears to be related to cumulative dose and the dose rate of administration. In our pre-clinical studies bisphosphonates delayed progression of osteolytic lesions in neuroblastoma tumors xenografted into immunocompromised mice while the combination of Zometa with low dose cyclophosphamide appeared to prolong overall survival. The primary aim of this study is to evaluate the maximum tolerated dose of Zometa combined with low dose oral cyclophosphamide in children with recurrent or refractory neuroblastoma. We will also evaluate the pharmacokinetics of Zometa in children with neuroblastoma and examine the effect of Zometa on markers of bone resorption, cytokines and bone-related growth factors. 