Eating broccoli and cauliflower regularly reduces the risk of deadly prostate cancer, say researchers.
Experts recommend five portions of fruit and vegetables a day
A study of 1,300 men found they were better than any other vegetable at protecting against aggressive tumours.
Writing in the Journal of the National Cancer Institute, the researchers said broccoli and cauliflower were known to contain anti-cancer compounds.
Experts advised the best way to reduce cancer risk was to eat a balanced diet, including lots of fruit and vegetables.
Several studies have shown a link between eating vegetables and a reduced risk of prostate cancer, but results have not been consistent and many have not specifically looked at deadly forms of the disease.
A team from the US National Cancer Institute and Cancer Care Ontario in Toronto carried out food questionnaires in a group of patients diagnosed with prostate cancer in a large screening trial.
Overall, they found that eating fruits and vegetables was not associated with decreased prostate cancer risk.
But greater consumption of dark green and cruciferous vegetables, especially broccoli and cauliflower, was associated with a decreased risk of aggressive prostate cancer.
A weekly serving of cauliflower was associated with 52% decreased risk of aggressive disease and similar amounts of broccoli cut the risk by 45%.
Spinach consumption also seemed to be associated with a reduced risk but the findings were not significant when the researchers looked at cancer which had spread beyond the prostate.
Study leader Dr Victoria Kirsch, Cancer Care Ontario, said: "Aggressive prostate cancer is biologically virulent and associated with poor prognosis.
"If the association that we observed is ultimately found to be causal, a possible means to reduce the burden of this disease may be primary prevention through increased consumption of broccoli, cauliflower, and possibly spinach."
However, she pointed out that men eating a lot of broccoli and cauliflower may be more healthy in general.
Prostate cancer kills one man an hour in the UK and 32,000 are diagnosed every year with the disease.
Dr Kat Arney, Cancer Research UK's senior science information officer, said: "When it comes to food, there is no one particular 'super' fruit or vegetable that will protect you from cancer.
"Experts have proven that the best way to reduce your risk of many cancers is to eat a healthy balanced diet.
"This should include at least five portions a day of a variety of fruit and vegetables including broccoli and cauliflower."
Chris Hiley, head of policy and research at The Prostate Cancer Charity said: "Broccoli and cauliflower have appeared in other studies as being potentially important in defences against prostate cancer.
"Whilst waiting for definitive evidence it's clear that men should be eating at least five portions of fruit and vegetables a day."