The Papanicolaou Smear is a screening test which detects premalignant lesions of the uterine cervix. By treating these lesions, cervical cancer can be evaded. In 1988, a cytological diagnosis which communicated a state of uncertainty in the atypicality of cervical cells was first created in the Bethesda Cytology Classification scheme. This diagnosis is now known as atypical squamous cells of undetermined significance (ASC-US) and still little is known about its natural history. / This paper analyzes the results of a longitudinal study incorporating repeated regular measurements of viral and cytological endpoints as well as lifestyle and behavioural aspects, to understand the natural history of an ASC-US Pap smear and identify determinants of ASC-US acquisition and clearance. / Overall, the median duration of ASC-US is short, and is dependent on the definition of clearance since most lesions regress to normal. The factors most predictive of ASC-US acquisition but not clearance relate to HPV infection.

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