US Edition Even though the men were not aroused when the measurements were taken, the device improved erectile function by up to 36 per cent.
A total of 21 "highly motivated" patients with an average age of 47 were recruited for the study, which involved using the extension device every day for six months.
Five patients failed to complete the course for different reasons, including "pain and penile bruising".
The men started out with an average flaccid penis length of 2.82 inches, which increased to 3.72 inches a year later. No significant increase in girth was reported.
A "satisfaction" rating system produced high scores for length during erection and sex life.
The research was reported today in the British Journal of Urology International.
Consultant urologist Dr Paolo Gontero, from San Giovanni Battista Hospital at the University of Turin, said: "Our study showed that the penile extender device produces an effective and durable lengthening of the penis, both in the flaccid and stretched state.
"If these results are confirmed by further research, we propose that the device should be used as a first-line treatment option for men seeking a penile lengthening procedure."
He added: "It should also be pointed out that the majority of men who seek help for a small penis fail to meet the clinical criteria for surgery. For example, only one of the men in our study had a flaccid penis length of less than four centimetres and American guidelines strongly discourage surgery if men exceed this measurement."
The Andro-Penis device consists of a plastic ring, two extender rods, and a silicon band to hold the penis in place.
The men were told to increase the traction from 600 grams in the first month to up to 1,200 grams in month six. Their progress was then checked for another six months.