MESOTHELIOMA is characterized by having a relatively long latency period, which means that symptoms do not develop until 10 to 50 years after exposure to asbestos. The average age at which symptoms usually begin to appear is around 60 years old. It is estimated that about 3,000 to 4,000 cases of mesothelioma are diagnosed each years in the US. The number of cases reported each year is increasing.