A simple sneeze may be a tell-tale sign of sexual arousal for a select few, research suggests.
Two British doctors investigated the phenomenon after reading of a middle-aged patient who had uncontrollable sneezing fits when he thought of sex.
They unearthed evidence, via internet chat rooms, of 17 others - of both sexes - with the same problem.
The Journal of the Royal Society of Medicine study suggests it may be down to a mix-up in brain circuitry.
The researchers, Dr Mahmood Bhutta and Dr Harold Maxwell, also uncovered three people who claimed to sneeze after orgasm.
Dr Bhutta, a specialist in ear, nose and throat medicine at Oxford's John Radcliffe Hospital, said the phenomenon could be more widespread than thought and might even be inherited.
He said: "It certainly seems odd, but I think this reflex demonstrates evolutionary relics in the wiring of a part of the nervous system called the autonomic nervous system.
"This is the part beyond our control, and which controls things like our heart rate and the amount of light let in by our pupils.
"Sometimes the signals in this system get crossed, and I think this may be why some people sneeze when they think about sex."
However, Dr Bhutta said there were only one or two references to the sex-and-sneezing link in published medical literature.
The most recent was a letter to the Journal of the American Medical Association in 1972 involving a 69-year-old man who complained of severe sneezing following orgasm.
However, Dr Bhutta said embarrassment or social inhibition may have prevented others from admitting the problem.
He added that internet chat rooms could be a potential new tool for investigating the incidence of unusual or embarrassing symptoms that patients may not feel appropriate to discuss with their doctor.
His papers also describes how genetics can explain why around one in four people sneeze as a response to sunlight.
Sneezing usually occurs in response to nasal irritation, triggering a reflex that expels air at speeds of around 150 kilometres an hour.
Mr Andrew McCombe, an ENT consultant at Frimley Park Hospital said there was a possible physiological explanation for a link between sexual arousal and sneezing.
He said during arousal the autonomic nervous system fires off signals to trigger changes in the genitals of both men and women.
However, the nose also contains erectile tissue, and this may also become engorged during arousal, triggering the need to sneeze.
There is a condition known as honeymoon rhinitis, in which men and women experience nasal stuffiness during sex.