Background: Asthma, endometriosis and menstrual pains affect large numbers of women. A possible association between asthma and endometriosis has been suggested, however, the relationship is unclear. Menstrual pains are very common and potential associations with asthma not known.

Aim: To study asthma and asthma-like symptoms in women as associated with endometriosis and menstrual pain.

Methods: We used data from the main and a women?s questionnaires of the Respiratory Health in Northern Europe study, including women (aged 40-65 years) from Aarhus, Gothenburg, Umea, Uppsala, Reykjavik, Tartu and Bergen. Current asthma was defined by asthma medication usage or asthma attacks in the past 12 months. Asthma symptoms score was defined as having ?3 asthma symptoms the last 12 months. Data were analysed using logistic regression adjusted for age, BMI and smoking.

Results: Among 4 826 study participants, 202 reported having endometriosis and 2 415 menstrual pains. Current asthma and ?3 asthma symptoms were reported by 14.8% and 24.8% of women with endometriosis compared to 9.2% and 17.3% without endometriosis, respectively. The associations of current asthma and asthma symptoms with endometriosis were statistically significant (OR 1.76, 95%CI 1.18-2.62; and OR 1.57, 95%CI 1.13-2.18, respectively). Similar associations were found for menstrual pain (current asthma: OR 1.48, 95%CI 1.23-1.80; ?3 asthma symptoms: OR 1.67, 95%CI 1.44-1.94).

Conclusion: Our study found that asthma and asthma symptoms were associated with both menstrual pain and endometriosis and that the associations with menstrual pains, which affect a large proportion of women, were almost as strong as associations with diagnosed endometriosis.