64Cu-DOTATATE PET/CT imaging 1 hour (h) post-injection (p.i.) is excellent for lesion detection in patients with neuroendocrine neoplasms (NEN). We hypothesized that the imaging time window can be extended up to 3h p.i. without significant differences in the number of lesions detected. Methods From a prospective study, we compared, on a head-to-head basis, sets of 64Cu-DOTATATE PET/CT images from 35 patients with NEN scanned 1h and 3h p.i. of 200 MBq 64Cu-DOTATATE. The number of lesions on both scans were counted and grouped according to organs or regions and compared with negative binomial regression. Discordant lesions (visible on the 1h or 3h p.i. 64Cu-DOTATATE PET but not the other) were considered true if found on simultaneous CT or later MR, CT or somatostatin receptor imaging. We measured lesion maximal standardized uptake values (SUVmax), reference normal organ or tissue mean SUV (SUVmean) and tumor-to-normal tissue ratios (TTN) calculated from SUVmax/ SUVmeanResults We found 822 concordant lesions (visible on both the 1h and 3h p.i. 64Cu-DOTATATE PET) and five discordant lesions of which four were considered true. One discordant case in one patient involved a discordant organ system (lymph node) detected on the 3h p.i. but not the 1h p.i. 64Cu-DOTATATE PET that did not alter the patient's disease stage (stage IV) because the patient had 11 additional concordant liver lesions. We found no significant differences between the number of lesions detected on the 1h and 3h p.i. 64Cu-DOTATATE PET. Throughout the 1-3 h p.i. imaging window, TTN (mean [95% confidence interval]) remained high in all key organs: Liver (1h p.i.: 12.6 [10.2; 14.9] , 3h p.i.: 11.0 [8.7; 13.4]), intestines (1h p.i.: 24.2 [14.9; 33.4], 3h p.i.: 28.2 [16.5; 40.0]), pancreas (1h p.i.: 42.4 [12.3; 72.5], 3h p.i.: 41.1 [8.7; 73.4]) and bone (1h p.i.: 103.0 [38.6; 167.4], 3h p.i.: 124.2 [57.1; 191.2]). Conclusion The imaging time window of 64Cu-DOTATATE PET/CT of patients with NEN can be expanded from 1h p.i. to 1-3 hours p.i. without significant differences in the number of lesions detected.