Asthma and hay fever are linked to irregular periods, research suggests.
The link was found in women aged 25-42
By surveying more than 8,500 women, a team found rates of asthma and allergy were far higher in women who had irregular rather than regular periods.
The Thorax study points to metabolic problems, such as insulin resistance, as an underlying cause for both.
The Norwegian team said their findings also added weight to the argument that female hormones might play a role in allergic diseases.
They ruled out the possibility that the trend might have been down to the allergy medication the women were using because the same link was also seen among women whose symptoms were not being treated.
Dr Cecilie Svanes and colleagues from the Haukeland Hospital in Bergen, Norway, asked the women about their respiratory health and menstrual cycles between 1999 and 2001.
About one in four women (23%) had irregular periods - 14% among those aged 25 to 42 and 37% among those aged between 43 and 54.
Among the younger women, irregular periods were most common in those who weighed the most or were the shortest or tallest.
Among the older women, they were associated with smoking and the timing of the menopause.
But after taking into account a number of factors, the researchers found that the rates of asthma and allergy were significantly higher in younger women with irregular periods than in those with regular periods.
They said irregular periods were often caused by polycystic ovary syndrome (PCOS) - a condition where the ovaries have abnormal cysts.
PCOS has been linked to a disturbance of normal metabolism that can be treated with drugs to aid the processing of sugars in the body, such as metformin.
Several studies have reported associations between lung function and resistance to insulin - the hormone that regulates blood sugar - which again suggests a metabolic disorder might be the underlying problem, the researchers said.
Dr John Moore-Gillon, president of the British Lung Foundation, said: "This new research is very interesting and we will follow any future developments closely.
"It is fascinating that this research suggests that hormones can play a part in a person being asthmatic or suffering with allergies.
"More than eight million people in the UK have a lung condition, including asthma and allergies, and it is vital that research such as this is funded to ensure they have the best quality of life and treatment."
Asthma UK said further research was needed into possible reasons for the association between asthma and irregular menstruation.