Objectives: To assess if the instillation of plasma rich growth factors (PRGFs) improves endometrial thickness (EMT) in frozen embryo cycles performed under hormone treatment where the endometrium was very thin (≤5 mm). Methods: First, a randomized controlled trial (RCT) was performed comparing women only receiving an increase in estrogen therapy (n = 9) and women receiving both the increase in estrogen therapy and three instillations of PRGF (n = 13). The second part of the study consisted of a prospective observational follow-up of the patients included in the RCT (for 1–3 months in the study group, and for 1–6 months in the control group). Results: In the RCT, there was an increase in EMT in both the PRGF and control groups. However, the increase was significantly higher in the PRGF group (1.30 ± 0.67 mm) compared to the control group (0.58 ± 0.51 mm). In the PRGF group, 23% achieved an EMT of 7 mm compared to 0% in the control group. There were 2 pregnancies in the PRGF group resulting from the 3 transfers performed in that group. No transfer was carried out in the control group. There was a significant increase in EMT 1–3 months after the PRGF cycle. The live birth rate per transfer was 20% in the cycles following the PRGF cycles, whereas it was 30% per starting woman. For women in the control group who later underwent PRGF, the LBR was 57.1%. Conclusions: PRGF instillation in cases of very thin endometrium increases EMT moderately. It is suggested that the beneficial effect of PRGF may persist for 1–3 cycles after instillation.