The first witnesses began testifying this week against the 70-year-old former general, who denies wrongdoing.
Mladic has previously complained of ill health and had two strokes during his 16 years as a fugitive, but has looked healthy and alert in court this week.
Mladic felt ill while his lawyer, Branko Lukic, was cross-examining witness David Harland, a former U.N. staffer in Bosnia.
He reached for his head and ran his hand through his hair before Presiding Judge Alphon Orie suspended the trial and called for a medic.
Lukic said Mladic was briefly unable to communicate with him, but then "composed himself" before being led out of court and taken to a hospital for tests.
No more details about Mladic's health were available by late afternoon.
Since his arrest in Serbia last year, Mladic has undergone surgery in The Hague for a hernia and been treated for other ailments including a kidney stone and pneumonia.
Last year, judges rejected a proposal by prosecutors to hasten Mladic's trial, fearing his health was too fragile for a lengthy case that could drag on for years.
Prosecutors had suggested trying Mladic first for the 1995 Srebrenica massacre, for which he is accused of genocide in the killing of 8,000 Bosnian Muslims, before starting a separate trial on charges of masterminding other Serb atrocities throughout Bosnia's 1992-95 war that left 100,000 dead.
The tribunal has had bad experiences previously with defendants in ill health. Former Yugoslav President Slobodan Milosevic died of a heart attack in his cell in 2006, forcing a premature end to his genocide case. Milosevic's trial had dragged on for nearly four years, partly because of his health problems and partly due to his political grandstanding.