Dietzen was a successful amateur winning the German cyclo-cross championship in 1980 and 1981 as well as winning in road races. He turned professional with the Swiss team Puch in 1982. He won the
cyclo-cross race the
Grand Prix Jean Bausch-Pierre Kellner that year. The following year he joined a
Spanish cycling team Teka with whom he would stay with for the rest of his career. In his first year with his new team he won the
Volta a la Comunitat Valenciana. The following year he became the champion of Germany in the road race and cyclo-cross as well his first stage victory in the
Vuelta a España. He won the stage to the
Lagos de Covadonga which is a very steep climb and a prestigious stage to win. He would finish that year's edition of the Vuelta third overall. The following year he was again cyclo-cross champion as well as the winner of the Vuelta a Cantabria. In 1986 he was again German road champion and won a second stage in the
1986 Vuelta a España. In 1987 he wore the leader's jersey in the
1987 Vuelta a España for five days before losing it to Herrera and then finishing the race second overall to
Luis Herrera of
Colombia. Dietzen finished the
1988 Vuelta a España second overall, this time to
Sean Kelly of
Ireland. In the
1989 Vuelta a España, Dietzen won a stage but several days later he crashed. He had ridden into a tunnel which was not illuminated and crashed, suffering career-ending injuries. He was only 30 years of age when the following year he stopped as a professional after not recovering. Seventeen years later, the
Supreme Court of Spain ordered the organisers of the Vuelta a España to pay damages to Dietzen. Dietzen obtained nearly all of his success in Spanish races with wins in
Vuelta a La Rioja,
Vuelta a Castilla y León, the
Setmana Catalana de Ciclisme and he finished in the top 10 of every Vuelta a España between 1984-1988, three of which were on the podium. ==Post-cycling career==