No study has so far investigated the relationship between aspirate color or degree of aspiration on the success of ethanol ablation (EA) of cystic thyroid nodules. We aimed to evaluate the efficacy of EA of benign cystic thyroid nodules and assess the relevant factors influencing the outcome. Over a 2-year period, 64 benign cystic thyroid nodules in 62 patients were treated with EA. Several factors related to EA efficacy were evaluated, including the cystic component volume, volume and color of aspirates, degree of aspiration, and volume of injected ethanol. In all cases, we performed ultrasound follow-up for at least 12 months after the last EA session to evaluate the collapsed cystic component. The 64 treated nodules had aspirate colors that were red bloody (n = 3), dark bloody (n = 31), brownish (n = 15), greenish-yellow (n = 13), and colorless (n = 2). The degrees of aspiration were scant (n = 8), mild (n = 3), moderate (n = 8), and complete (n = 45). There was successful collapse of the cystic component after initial EA in 52 cases, but a repeat EA was employed in 12 failed cases. Statistical analysis showed that the degree of aspiration and color of aspirates correlated significantly with the success of EA. The results of this study suggest that complete aspiration of cystic contents was the most important factor in the efficacy of EA of benign cystic thyroid nodules, and greenish-yellow contents were closely related to scant or mild aspiration.