A poisonous substance used as weed killer and notorious as a weapon for murder mystery writers may be used to treat leukaemia, researchers believe.
There is up to a 80% five-year survival rate for APL with chemotherapy
The Iranian team said arsenic trioxide could be used as first-line treatment for acute promyeloctytic leukaemia.
They found that after two courses of treatment more than 90% of the 63 patients were in complete remission.
But UK experts said traditional chemotherapy was still likely to be used in western countries.
Arsenic trioxide is already used on patients as second-line of treatment for the leukaemia, which affects 20,000 people across the world each year.
But initial treatment involves chemotherapy plus Atra, a vitamin A-based substance. It has a five-year survival rate between 60% and 80%.
The arsenic trioxide research by scientists at Tehran University of Medical Sciences reported 88.5% of patients were still alive after 34 months.
And the study, revealed at a European Organisation for Research and Treatment of Cancer conference in Geneva, also said it could be effective in targeting other cancers such as multiple myeloma.
Arsenic trioxide works by causing changes in cancer cells which induce apoptosis - programmed cell death.
The researchers said it also appeared to correct the gene responsible for making a flawed protein that causes APL.
It is not the first time arsenic trioxide has been found to treat leukaemia.
A Chinese study earlier this year found the arsenic used in combination with Atra was an effective treatment.
Lead researcher Dr Ardeshir Ghavamzadeh said: "What this means is that we now have the possibility of offering APL patients a new first-line treatment that avoids conventional chemotherapy.
"It also means that if we have this drug and other effective drugs such as Atra available as well, most patients will be able to avoid the need for bone marrow transplants."
But Dr David Grimwade, of Guy's, King's & St. Thomas's School of Medicine, said arsenic trioxide was unlikely to become a first-line treatment in the UK.
He said doctors were still a "little nervous" about possible liver and cardiac complications from using arsenic trioxide.
"Chemotherapy is a very effective form of treatment.
"In western countries, where there is the resources and facilities to put patients through long and intensive course of chemotherapy, I can't see arsenic being used for first-line treatment."
However, he acknowledge it may well be more useful in the treatment of the elderly, who often die earlier in the treatment process.
And he added: "What is interesting about this study is that it directly attacks the underlying causes of the leukaemia, which is particularly relevant in developing countries where there are less resources."
Ken Campbell, clinical information officer at the Leukaemia Research Fund, said it was an "interesting study".
"It is possible that arsenic trioxide may be found to be of particular value to patients for whom standard chemotherapy may be unsuitable."