Vanhal was a student of Dittersdorf and was considered one of the best symphonists of his day. His music was popular in its time, but like that of many composers it fell out of favor as time moved on. Vanhal suffered some sort of mental breakdown for a year or two, and some say he never fully recovered. It is possible to hear traces of his illness in his music: the ratio of minor to major keys in his works is much higher than with other composers of the day, and he had a fondness for dissonant chromatic "sighing" motifs. One of his G-minor symphonies has been ranked with those in the same key by Haydn and Mozart. In his later years he turned to ligher, less substantial works for the amateur player, mostly keyboard variations.

Source: New Grove Dictionary of Music and Musicians, my teachers.