after being assistant conductor at the bayreuth festival his first important opportunity came in 1969 when the conductor william steinberg became unwell during a concert with the boston symphony orchestra. tilson thomas took over conducting the rest of the concert. he stayed with the orchestra as assistant conductor until 1974, making several recordings with them. he was conductor of the buffalo philharmonic orchestra from 1971 to 1979. during this time he conducted a series of young people's concerts with the new york philharmonic. from 1981 to 1985 he was the main guest conductor of the los angeles philharmonic orchestra. tilson thomas started a new orchestra, the new world symphony, in miami, florida in 1987, to give talented young people a chance to play with a good orchestra. from 1988 to 1995, he was principal conductor of the london symphony orchestra, and since 1995 he has been their main guest conductor. in 1995, he became conductor (music director) of the san francisco symphony orchestra.