OBJECTIVES: In an earlier interview study, we found that more men with familial schizophrenia had undergone inguinal hernia operation, than men with sporadic schizophrenia. However, there are no other studies published specifically on inguinal hernia and schizophrenia. Therefore, the aim of this study was to carry out a Swedish register-based cohort study on the association between inguinal hernia and schizophrenia or related psychosis. METHODS: Data from the Total Population- and Medical Birth-Registers were used to create a cohort of all individuals born in Sweden 1987-1999 (n=1 406 168). The cohort individuals were linked with the In- and Out-patient Registers and followed from birth to 2015 to identify onset of schizophrenia, schizoaffective disorder and inguinal hernia. Cox proportional hazards regression models were used to assess the association between inguinal hernia before age 13 and risk of developing schizophrenia or schizoaffective disorder during a follow-up from age 13. RESULTS: Inguinal hernia before age 13 was identified in 21 095 individuals, and during the follow-up in total 1314 individuals developed schizophrenia or schizoaffective disorder. The risk of schizophrenia or schizoaffective disorder was higher among individuals with inguinal hernia before age 13, than among individuals without such a diagnosis, especially among the men [adjusted hazard ratio (95% confidence interval); all: 1.44 (1.01-2.06), p=0.0452, men: 1.46 (1.01-2.12), p=0.0460, women: 0.56 (0.14-2.27), p=0.4173]. CONCLUSIONS: This study shows that early-onset inguinal hernia is associated with increased risk of developing schizophrenia or schizoaffective disorder, especially in men. Such an association may point to a common biological basis for the development of inguinal hernia and schizophrenia or related psychosis.