Dr. Lange is a professor of Internal Medicine at the Academic Medical Centre of the University of Amsterdam, where he originally became heavily involved in the clinical care of HIV-infected patients as early as 1982. The Academic Medical Centre has a pioneer-function in this field in the Netherlands, and about 60% of Dutch PWAs were initially referred to it.

In 1984, Dr. Lange became involved in providing clinical guidance to the Amsterdam Cohort-Study of HIV Infection and AIDS. In 1985, he started laboratory investigations in the Human Retrovirus Laboratory of the University of Amsterdam, which led to his PhD thesis, "Serological Markers in HIV Infection."

Dr. Lange has served as director of the National AIDS Therapy Evaluation Centre (NATEC) since 1990, coordinating clinical trials in the field of HIV-infection and its secondary complications in about twenty Dutch hospitals caring for HIV-infected people.

From 1992 until 1995 ran such efforts on a global scale as a chief of the World Health Organization's Clinical Research and Product Development branch of the Global Programme on AIDS. Under Dr. Lange, the Research and Development unit worked to promote and initiate prevention, diagnosis and treatment projects throughout the world. Such projects continue in Argentina, Brazil, Ivory Coast, Mexico, Tanzania, Thailand, Uganda, Uruguay, and Venezuela.

In January of 1995, Dr. Lange returned to his alma mater, the University of Amsterdam, to take up the position he currently holds as a professor of Internal Medicine, researching and teaching about viral diseases in particular.