Researchers have carried out several types of epidemiologic studies in humans to investigate the possibility of a relationship between cell phone use and the risk of malignant (cancerous) brain tumors, such as gliomas, as well as benign (noncancerous) tumors, such as acoustic neuroma (tumors in the cells of the nerve responsible for hearing that are also known as vestibular schwannomas), meningiomas (usually benign tumors in the membranes that cover and protect the brain and spinal cord), and parotid gland tumors (tumors in the salivary glands) (3).
Numerous peer-reviewed studies have shown an association between cell phone use and the development of brain tumors. According to a Mar. 2008 meta-analysis of cell phone studies there is a "consistent pattern" connecting cell phone use and an increased risk of developing glioma, a type of brain tumor.  A Mar. 31, 2009 study found that long term cell phone use (10 years +) "approximately doubles the risk" of being diagnosed with glioma on the same side of the head where the cell phone is held.  In Apr. 2013 another study of Swedish cell phone users also found an association between cell phone use and the development of glioma and acoustic neuroma - a benign tumor formation on the nerve near the ear.  That study’s conclusions were confirmed by a different study in Apr. 2014.  Other studies published from 2005-2013 have similarly concluded that there is an association between cell phone use and increased risk of developing brain and head tumors.    
the Environmental Working Group is suspending publication of the EWG guide to cell phones until the FCC makes the responsible decision to require cell phone makers to generate and disclose data about device and network emissions under real-world conditions. We strongly believe that as cell phones become more powerful and ubiquitous, it is critical that people have a right to know how much radiation they can expect their cell phones to generate. As things now stand, the FCC’s cell phone safety rules are as obsolete as the StarTac.
Cancer is the obvious start. An early concern with mobile technology was clusters of the disease around those living near phone masts. One study in Israel found a 4.5-fold increase in cancers of all kinds in the immediate vicinity of a mast (Int. J. Cancer Prev., 2004). In 2009, a Korean team of researchers carried out a pool analysis of the results of 23 studies, which involved almost 38,000 subjects.