Purpose: The diagnosis of prostate cancer and the following treatment does not only affect the patient, but also his partner. Partners often suffer even more severely from psychological distress than the patients themselves. This analysis aims to describe the quality of life (QoL) after the cancer diagnosis over time and to identify the effects of possible predictors of partners' quality of life in a German study population. Data and methods: Patients with localised prostate cancer and their partners were recruited from a prospective multicenter study in Germany, the Prostate Cancer, Sexuality, and Partnership (ProCaSP) Study. At five observation times during the follow-up period of 2 years after diagnosis, QoL (EORTC QLQ-C30) and personal, social, and cancer-related health factors as well as adaptation and coping factors of 293 couples were observed and analysed with mixed effects analysis. Results: The men's prostate cancer diagnosis had a small, but significant impact on their partner's QoL. However, QoL of partners was most affected by the partners' own physical health and psychological condition, time, and their relationship quality. Conclusion: The finding that average QoL increased again 3 months after diagnosis and later should give partners faith and hope for the future. The identified most important predictors of partners' QoL are potentially susceptible to intervention, and further research on target groups in special need of support and on adequate interventions is needed.