Parkinson’s disease (PD) patients can benefit from regular physical exercises which may ease their symptoms and can slow down the progression of the disease. Motion-based video games can provide motivation to carry out the often repetitive exercises, as long as they establish a suitable balance between the level of difficulty and each player’s skills. We present an adaptive game system concept, which is based on separate difficulty parameters for speed, accuracy and range of motion. We then describe the heuristic performance-evaluation and adjustment mechanisms in a prototypical implementation which was applied in a case study with three PD patients over a period of three weeks. Results indicate that the system facilitated a challenging yet suitable game experience and a detailed analysis of the results informed a number of follow-up research questions for future research.