Autosomal recessive (AR) STAT1 deficiency is a severe inborn error of immunity disrupting cellular responses to type I, II, and III IFNs, and IL-27, conferring a predisposition to both viral and mycobacterial infections. We report the genetic, immunological, and clinical features of an international cohort of 32 patients from 20 kindreds: 24 patients with complete deficiency and eight patients with partial deficiency. Twenty-four patients suffered from mycobacterial disease (BCG =13, environmental mycobacteria =10, or both in one patient). Fifty-four severe viral episodes occurred in 16 patients, mainly caused by Herpesviridae viruses. Attenuated live MMR and/or VZV vaccines triggered severe reactions in the five patients with complete deficiency. Seven patients developed features of hemophagocytic syndrome. Twenty-one patients died, and death was almost twice as likely in patients with complete STAT1 deficiency than in those with partial STAT1 deficiency. All but one of the eight survivors with AR complete deficiency underwent hematopoietic stem cell transplantation (HSCT). Overall survival after HSCT was 64%. A diagnosis of AR STAT1 deficiency should be considered in children with mycobacterial and/or viral infectious diseases. It is important to distinguish between complete and partial forms of AR STAT1 deficiency, as their clinical outcome and management differ significantly.