Background Endometriosis is an invalidating gynaecological condition in women of reproductive age, and a frequent cause of infertility. Unfortunately, the condition is characterized by a long interval between onset of symptoms and diagnosis. GPs in the Netherlands are educated to provide basic gynaecological care and serve as gatekeepers for specialist medical care. Therefore, it is of great importance that they recognize signs and symptoms possibly caused by endometriosis to initiate adequate actions. Objective The main objective of this study was to identify barriers and facilitators to the timely diagnosis of endometriosis from the GPs’ perspective. Methods Semi-structured focus group discussions with GPs were organized throughout the Netherlands. The participants were encouraged to brainstorm about their perspective on daily practice regarding endometriosis and suggestions for interventions to enable early diagnosis and treatment. Analysis was based on grounded theory methodology. Results Forty-three GPs participated in six focus groups. Analysis of the transcripts revealed relevant determinants of practice in four main themes: professionals’ experience and competence, patient characteristics, guideline factors and professional collaboration. A lack of knowledge and awareness appeared to result in a low priority for establishing the diagnosis of endometriosis, especially in young women. Infertility, patient engagement and a recent serious case or training facilitated referral. Conclusion Several factors in daily primary health care contribute to the diagnostic delay in endometriosis. Future interventions to reduce this delay may be aimed at increasing awareness by means of education, incorporating the subject into national clinical guidelines and improvements in interdisciplinary collaboration.