The detection of diseases can exhibit seasonal fluctuations. This can be studied in cervical smears. Over a 9-year observation span (January 1983–January 1992) a series of 504,093 cervical smears obtained from a routine cytology laboratory in The Netherlands were examined for infections (monilia, trichomonas, actinomyces, human papilloma virus [HPV], chlamydia, and herpes) as well as for mild, moderate, and severe dysplasias, carcinoma in situ, and squamous carcinoma. Statistical analysis (principal component analysis) demonstrates clear seasonal rhythms in the detection of infections as well as in precursor lesions. These findings suggest that we are dealing with “true” detection rhythms. For the detection of (pre)malignancy and HPV, yearly fluctuations in women being screened might be the explanation for our observations. Diagn. Cytopathol. 1997;17:452–455. © 1997 Wiley-Liss, Inc.