52 dairy herds, infected with Mycobacterium avium subsp. paratuberculosis (Map), were submitted to repeated bulk milk sampling (an average of 3.5 samples/herd). On 183 samples, tested by culture and nested-PCR, 20 (11%) resulted positive for Map. 11 herds on 52 controlled (21.2%) registered at least one positive sample. The positivity appears strongly correlated to the herd prevalence. In the risk factor analysis for milk contamination, both the paratuberculosis herd prevalence and the hygienic measures to control the fecal contamination were taken into consideration; the risk of milk contamination appears directly related both to the infection prevalence in the herd and to udder hygiene. In contrast, neither the ideal hygienic measures in routine milking, nor proper milk filtration were effective in preventing the presence of Map in milk.