Purpose <p>Acute Myocarditis (AM) was recently shown to be detected by the Somatostatin Positron Emission Tomography (PET) criterion of &gt; 18 cm³ Myocardial Uptake Volume (MUV), a sign of significant inflammatory cell infiltration. This study characterizes patients for whom this criterion persists 4 to 5&#xa0;months after AM.</p> Methods <p>Cardiac Magnetic Resonance (CMR) and [⁶⁸&#xa0;Ga]Ga-DOTA-TOC PET data from 27 AM patients (2 women, median age 26.5&#xa0;years [interquartile range: 21.9–31.9]) were analyzed at the acute phase and at a 4.5 [4.3–5.0] month follow-up.</p> Results <p>Eleven AM patients (41%) still had &gt; 18 cm³ MUV at the follow-up (PET +). The left ventricular ejection fraction (LVEF) was correlated with MUV at baseline (<i>p</i> = 0.011) and follow-up (<i>p</i> = 0.001) and was lower at follow-up in PET + (52.9 [48.6; 55.0] %) than in the other patients (56.0 [54.3; 57.8] %, <i>p</i> = 0.001). However, this poorer recovery of the PET + LVEF was associated with two MUV evolution profiles evocative of different mechanisms: (i) a prolonged active disease in the 5 PET + patients for whom the MUV increased at follow-up, in association with a slight decrease in LVEF (p = 0.08), and (ii) a more severe initial insult in the 6 other PET + patients for whom the MUV decreased at follow-up with concomitant increases in LVEF (<i>p</i> = 0.028) but these improvements started from much worse baseline LVEF and MUV (respectively, <i>p</i> = 0.022 and 0.003 vs. the other patients).</p> Conclusions <p>Somatostatin PET monitoring of AM unveils numerous patients with signs of a persistent inflammatory cell infiltrate 4 to 5&#xa0;months after acute myocarditis. This persistence is associated with poorer recovery of cardiac function and is seemingly due to active inflammation that is either more severe at baseline or continues to expand over the following months.</p> <p><b>Trial registration number:</b> NCT03347760 on clinicaltrials.gov. </p> <p><b>Registration date:</b> 22–11-2017.</p>