The shrinking manhood warning came from the Boston University School of Medicine whose preliminary findings from a study of 200 men were revealed in the London Observer newspaper on Sunday.
Smoking affects the penis in the same way that it does the heart, the researchers said.
It damages the blood vessels and inhibits blood flow. This in turn affects elastin, the substance which is believed to govern a man's ability to have an erection. British anti-smoking campaigners welcomed the findings as a way of shocking smokers into stopping.