BACKGROUND: Kangaroo mother care (KMC) is recommended as a beneficial intervention to promote the wellbeing of preterm infants. This study evaluated the effect of KMC on anthropometric indexes in preterm newborns, with and without telephone advice (TA). METHODS: At a tertiary center, in a single-blinded, randomized controlled trial (RCT), one hundred and five discharged preterm infants from the neonatal intensive care unit (NICU) were randomly allocated into two experimental groups: KMC, KMC with telephone advice (KMC-TA), and conventional care (CC) (35 birthing parent-infant pairs in each group). In two experimental groups, a research assistant trained participants on how to do KMC at home for a maximum of three times, at least 3-4 h daily, during a one-month period. In the KMC-TA group, participants were counseled on KMC by phone, twice a week. In CC group, routine care was provided. Prior to, and at the end of the intervention, anthropometric indexes including the weight, height, head and chest circumferences of neonates were measured in the three groups. The ANOVA, Kruskal-Wallis, Chi-square, Fisher's exact test, and Bonferroni was used to analyse the data. RESULTS: After one month, the mean weight of neonates was significantly greater in the two experimental groups when compared to those the CC group (p = 0.006). No significant differences were observed in other anthropometric indexes. CONCLUSIONS: Short-term implementation of KMC has a positive effect on preterm infant weight gain. Optimal implementation strategies for KMC are required, and future research may usefully inform these. TRIAL REGISTRATION: This trial was registered in the Iranian Registry of Clinical Trials with code IRCT201306082324N11 on 28/4/2014. URL of registry https://irct.behdasht.gov.ir/trial/1966 .