Aim: This study describes three reported outbreaks of influenza H1N1 2009 in Spanish prisons between July and September 2009.Methods: An outbreak was defined as the appearance of three or more cases with influenza symptoms and with an epidemiological link in the same module of a prison.The outbreaks were reported using a specific questionnaire.The analysis, which used variables of gender, age, date of notification, duration of outbreak, risk factors and clinical features are presented as absolute numbers, percentages and attack rates, while study of the diffusion of the illness is expressed as epidemic curves.Results: Three outbreaks were reported at the prisons of Alcala-Meco (85 affected males), Pamplona (18 affected males) and Jaen (12 affected females) with an overall attack rate that ranged from 7.1% to 17.9%.Duration of the outbreaks was between 8 and 35 days, and the average duration of the illness itself was 3 days.Only 4 inmates were admitted who were later given discharged for recovery.The men were younger (p<0.001).The epidemic curves of the outbreaks did not show any clear propagation patterns.Discussion: The attack rates are highly variable although they are lower than other community outbreaks.Symptoms were slight and lethality was zero.The women's age was significantly greater than that of the men, although it is practically the same amongst the prison population.Low morbidity was very probably the cause of the reduction in consultations of the persons affected and the consequent underestimation of the rates.