Objective: To examine the separate and joint effects of male and female fertility problem (FP) stress and the source of stress (e.g., personal, social, marital) on treatment outcome. Design: Prospective, epidemiological cohort design. Setting: Fertility clinics in Denmark. Patient(s: Eight hundred eighteen couples who were about to begin a new course of treatment. Intervention(s): An FP stress inventory was administered at the start of treatment, and the treatment outcome was evaluated 12 months later. Main Outcome Measure(s: Number of treatment cycles in 12-month study period and treatment outcome (i.e., success, no success). Result(s: Fertility problem stress was associated with a poorer treatment outcome in women (pooled within-groups [WGr] correlation,WGr = .517) and men (WGr = .392) with the effect significantly more pronounced for women (z = 3.19, P