The study aimed at assessing the dynamics and risk factors of intramammary infection (IMI) during the dry period in herds under selective dry cow therapy (DCT). Quarter milk samples from all cows of the 28 enrolled herds were collected at drying-off and on day 3 after calving to be submitted to microbiological procedures. Individual cow recording forms and a herd questionnaire were used to collect potential risk factors for new IMI in untreated cows only. Analysis was done using a multivariate model (GLIMMIX macro) at quarter level. 33.0 and 23.6% of the quarters were infected at drying-off and 3 days after calving, respectively. 19.0% had a new IMI during the dry period (20.3% and 16.5% in untreated and treated cows, respectively). In untreated cows, 4 risk factors of new IMI were identified (P< 0.10): the cow infection status at drying-off for the other quarters than the studied one; a long preceding lactation length; a long dry period length; and the absence of concentrate suppression before drying-off. Incidence of IMI during the dry period was quite high compared with previous studies, especially in untreated cows. The efficiency of selective DCT towards new IMI could be improved if untreated cows were quite all uninfected. Therefore, criteria to select relevantly uninfected cows to be left untreated should be further investigated.