Little is known about prosocial behaviours in children of young mothers. This longitudinal study involved 44 young mother (age less than 20 years at birth of first child) families and 44 older mother families, who were carefully matched for child age and gender, as well as for family structure (number of children, lone-parent status) and socio-economic background (maternal education, head-of-household unemployment, low family income). At the first time-point (M child age = 2.45 years), mothers listed family members or friends who provided practical and/or emotional support; videos of mother-child interaction in play and tidy-up were coded for positive and negative parenting. Two years later (M child age = 4.32 years), children's prosocial behaviours were assessed via (i) an experimental task of willingness to share and (ii) video-based ratings of sharing and helping during dyadic play with friends. "T"-tests showed few group differences in prosocial behaviours. Regression analyses showed that, for young mother families only, reduced negative parenting mediated associations between support from friends and preschoolers' willingness to share. Support from friends, via parenting, may promote willingness to share in children born to young mother families. (Contains 3 tables and 2 figures.)