Gavin McCann fired the Trotters into a 25th minute lead at the Reebok Stadium with his second goal of the European campaign, but Simon Vukcevic's 69th minute equaliser earned the Portuguese side a draw and a priceless away goal.
Bolton suffered a blow in the pre-match warm up when goalkeeper Jussi Jaaskelainen was injured and had to be replaced by Ali Al-Habsi.
Kevin Davies went close to opening the scoring in the fourth minute when he was found unmarked at the back post by Nicky Hunt's cross, but his shot was blocked by some desperate defending.
Sporting goalkeeper Pedro Rui Patricio looked uneasy with Bolton's direct style of play and was lucky to escape action after clattering into the onrushing Heidar Helguson outside his penalty are soon after.
However, he was not so lucky in the 25th minute when Bolton took the lead with a well-worked goal.
Davies flicked on a Hunt long ball and, after Matt Taylor's shot was blocked, the ball fell to McCann, who coolly dispatched a side-footed effort home from 15 yards.
Sporting gradually came out of their shell and playmaker Joao Moutinho fired a shot from inside the penalty area narrowly over the crossbar.
But Bolton began the second half back on the front foot and should have been 2-0 ahead in the 54th minute.
Davies again flicked the ball on for the arriving Helguson, but the Icelandic striker could only clatter the bar with a header from inside the six-yard box.
Wanderers were made to pay for their profligacy when Sporting restored parity moments after Leandro Romagnoli had hit the bar with an angled shot.
Rodrigo Tiui got his head to another long ball forward and Vukcevic dispatched a thunderous left-foot volley past the helpless Al-Habsi from just outside the penalty area.
Both sides traded half-chances in the closing stages in an end-to-end tie which remains very much in the balance ahead of the second leg in Portugal next week.