Essential thrombocythemia (ET) is a heterogeneous disorder. For example, the growth of erythropoietin-independent erythroid colonies, termed "endogenous erythroid colonies (EECs)", has previously been observed in only 50% of ET patients. We have recently described the overexpression of a hematopoietic receptor, PRV-1 (polycythemia rubra vera-1), in patients with polycythemia vera (PV). Here, we compare PRV-1 expression and EEC formation in a cohort of 30 patients with ET; 50% of the ET patients in our cohort displayed EEC growth. Likewise, 50% of the ET patients overexpressed PRV-1. Remarkably, only the 15 ET patients displaying EEC growth showed elevated PRV-1 expression, while the 15 EEC-negative ET patients expressed normal PRV-1 levels. It has previously been reported that EEC-positive ET patients develop PV during long-term follow-up. Here, we show that 40% of the PRV-1-positive patients develop symptoms of PV during the course of their disease. In contrast, none of the 15 PRV-1-negative patients displayed such symptoms (p=0.017). Moreover, PRV-1-positive patients had a significantly higher number of thromboembolic or microcirculatory events (p=0.003). We propose that PRV-1-positive ET comprise a pathophysiologically distinct subgroup of patients, one that is at risk for the development of complications and for the emergence of PV.