New Leyton Orient manager Alberto Cavasin saw his side downed by a Christian Burgess goal as Portsmouth recorded a 1-0 win in London.
Italian Cavasin became the eighth man to take on the manager's role since president, and fellow countryman, Francesco Becchetti took over the East London club in July 2014.
But he could not arrest a worrying sequence that has seen Orient win just once in eight matches in Sky Bet League Two.
Defender Burgess stole up unmarked to powerfully head home a corner from Carl Baker on 57 minutes that was to settle the contest.
The opening half was a period of half-chances, although Pompey did have the ball in the net when Conor Chaplin's first-time flick into the net was ruled out for offside.
The visitors were given an excellent opportunity to break the deadlock when O's keeper Alex Cisak attempted to send a short pass to a defender, but only succeeded in picking out Chaplin. The Orient shot-stopper saved his blushes by saving the striker's effort.
Orient felt they should have had a penalty when Paul McCallum went to ground after 12 minutes but referee Christopher Sarginson waved away their frantic appeals.
Having fallen behind, Orient were unable to break down a well-organised Portsmouth side who almost doubled their lead when O's full-back Nicky Hunt headed a cross against his own post before the ball was scrambled clear.