Two clubbers have died and another is seriously ill in hospital, as police investigate whether they might have taken a rogue batch of ecstasy.
All three men had attended Alexandra Palace in north London, which regularly plays host to dance music events.
The dead men, aged 20 and 21, were admitted to hospital just after midnight and died hours later.
Police said anybody who took drugs at the venue and now felt unwell should seek medical attention at once.
A spokesman said there were indications that all three might have consumed what they believed to be MDMA, the active ingredient of ecstasy.
But he added that it would be impossible to know for sure until toxicology tests had been carried out.
The 20-year-old victim had attended a drum and bass and "dubstep" event on Friday night that continued into Saturday.
The other man who died, and the man, 20, who is in hospital, went to a separate dance event featuring Radio 1 DJ Pete Tong, which continued from Saturday to Sunday.
Det Insp Rita Tierney said: "Although it is too early to say what caused these men's health to deteriorate, we are investigating the possibility that illegal drugs may have been involved.
"If you have taken what you believed to be MDMA, or any other substance during this weekend's events at Alexandra Palace and are now feeling unwell, I would strongly urge you to attend your nearest hospital as soon as possible."
A London Ambulance Service spokesman said: "We were called at 3.40 on Saturday morning and again at six o'clock this morning about patients taken ill at Alexandra Palace.
"In both cases the patients were treated by private medical providers at the venue."
Alexandra Palace could not be contacted for comment.
