It cost $30 million, but the World Health Organization's study of tumours and cellphone use still doesn't provide definitive answers.
Links:
[1] http://admin.indiaenvironmentportal.org.in/feature-article/dont-hang-cellphones-dont-cause-tumours-probably
[2] http://admin.indiaenvironmentportal.org.in/category/author/nic-fleming
[3] http://admin.indiaenvironmentportal.org.in/category/journal/new-scientist
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/cancer
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/radiation
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/brain-cancer
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/research
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/who
[11] http://admin.indiaenvironmentportal.org.in/category/thesaurus/telecommunication