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Background: The most frequent noncutaneous site of involvement of chickenpox is the central nervous system (CNS) and complications include cerebellar ataxia, encephalitis, and meningitis., Objectives: We have recently observed an unusually high number of children with chickenpox CNS complications in our university children's hospital. A study to evaluate the incidence of these complications over time in children living in Tuscany was carried out., Study Design: We evaluated all cases of chickenpox and chickenpox complications leading to hospitalization in children aged 1 month-14 years reported to the Tuscany public health centre between 1997 and 2004. The International Classification of Disease Ninth Revision-CM hospital discharge diagnostic codes and medical records were used., Results: The incidence (95% confidence interval) of CNS complications/1000 chickenpox cases was stable between 1997 and 2001 [1997: 0.80 (0.29-1.74); 1998: 0.73 (0.29-1.50); 1999: 0.67 (0.25-1.46); 2000: 0.56 (0.15-1.44); 2001: 0.59 (0.16-1.50)] but increased significantly (chi(2) for trend: 9.401; p=.0021) in 2002 [1.56 (0.83-2.66)], in 2003 [1.73 (0.95-2.90)] and in 2004 [1.51 (0.74-2.27)]. Non-CNS complications remained stable over time., Conclusions: Possible factors biasing the result were taken into account. Reasons of increased CNS complications remain unknown, but the possible emergence of a particularly neurotropic strain of varicella-zoster virus should be further investigated.