|Date 28 April 1991||Course length 5.040 km (3.132 mi)|
|Official name XI Gran Premio di San Marino|
Location Autodromo Enzo e Dino Ferrari, Imola, Emilia-Romagna, Italy
Course Permanent racing facility
Distance 61 laps, 307.440 km (191.034 mi)
The 1991 San Marino Grand Prix was a Formula One motor race held at Imola on 28 April 1991. The 61-lap race was the third round of the 1991 Formula One season and was won by Ayrton Senna driving a McLaren-Honda.
Stefan Johansson was replaced at AGS by F1 debutant Fabrizio Barbazza. The team were under new management and their car colour was now red and blue having been usually white.
Ayrton Senna claimed his 55th pole position from Riccardo Patrese, Alain Prost, Nigel Mansell and Gerhard Berger. However, the formation lap saw two dramatic incidents - Prost spun off the track at Rivazza Turn, followed by Berger, who was able to continue. However Prost stalled the engine and did not take the start.
At the lights, Patrese took the lead ahead of Senna, whilst Mansell, already slow off the line with gearbox problems, retired at the end of lap 1 after a collision with Martin Brundle. He was followed out by Nelson Piquet who spun on lap 2 and Jean Alesi who spun off on lap 3 attempting a rather foolhardy pass on Stefano Modena.
In a strong lead, Patrese pitted for originally what appeared to be an early stop to slicks turned out to be more serious - a misfire with a faulty camshaft sensor. He restarted last before retiring for good 9 laps later.
Berger was catching Senna, lapping 1.5 seconds quicker than his teammate. The lead was soon down to 5 seconds, with Modena a superb third from Satoru Nakajima and the two Minardis of Pierluigi Martini and Gianni Morbidelli.
Both McLarens pitted for tyres with Senna maintaining his lead. Just after setting fastest lap, Berger was delayed in traffic, held up by the trio of Maurício Gugelmin, Julian Bailey and Thierry Boutsen. Bailey himself moved past Andrea de Cesaris into 6th, whilst Nakajima retired with transmission problems.
Ivan Capelli spun into retirement from 4th to hand over to JJ Lehto's Dallara. Modena retired with transmission problems which meant that behind the two dominant McLarens, the order was now Roberto Moreno, Lehto, Eric van der Poele in for the little Modena team and Martini's Minardi.
Moreno's gearbox broke on lap 52 causing him to retire, whilst Senna was having problems with oil pressure caused by the special high-torque Honda V12. Berger put in a series of fastest laps to cut Senna's lead to just 1.7s at the line.
Lehto was overjoyed to gain the first podium place of his career for Dallara, with Martini 4th. Van de Poele's drive ended when a fuel pump broke on the last lap - he was classified ninth overall. The Lotus drivers of Mika Häkkinen and Bailey took 5th and 6th, both scoring their first world championship points, an unexpected result for the troubled team since their cars had barely managed to enter the grid with Häkkinen 25th and Bailey 26th.