Abstract Context In patients with acromegaly on long-term treatment with long-acting somatostatin receptor ligands (SRLs), the time of blood collection for IGF-I measurement after injection is not well defined. Objective We aimed to assess serum IGF-I dynamics and variability in SRL-treated patients compared with surgically cured patients and healthy controls. Methods Thirty patients under SRLs considered controlled based on a normal previous IGF-I level, 10 patients cured by pituitary surgery, and 7 healthy subjects underwent 4 weekly IGF-I determinations. Results In SRL-treated patients, the IGF-I SDS (mean ± SD) was higher just before injection (0.34 ± 0.66) than at Day 7 (−0.33 ± 0.61; P = 0.0041) and Day 14 (−0.23 ± 0.60; P = 0.047) after injection, but it did not significantly vary in cured patients and healthy controls. The IGF-I CV was higher in SRL-treated patients than in cured patients or healthy controls (14.4 ± 7.6% vs 7.9 ± 4.4% and 8.3 ± 3.2%, respectively; P < 0.05 for both). Among SRL-treated patients, IGF-I CV was higher in “nonoptimally controlled patients”—i.e., patients with at least one elevated IGF-I value out of 4 (n = 9) compared with “optimally controlled” patients for whom all 4 IGF-I SDS values were < 2.0 (21.3 ± 9.3 vs 11.6 ± 6.0%; P = 0.0019). The latter did not differ from surgically cured patients and healthy controls. The measurement at the farthest distance from the SRL injection was the most predictive of patients with nonoptimally controlled disease. Conclusion In patients treated with long-acting SRLs, IGF-I sampling at the farthest distance from SRL injection is the most informative and best predictor of optimal disease control.