The aim of the present study was to assess the stability of alexithymia in adolescents and the effects of parental factors and social support thereon. The sample comprised 315 late adolescents, of whom 259 were female and 56 male. At baseline, the mean age of the subjects was 19 years (range 17–21 years). The follow-up period was 4 years (2008–2012). The 20-item Toronto Alexithymia Scale (TAS-20) was used for the assessment of alexithymia both at baseline and follow-up. The Multidimensional Scale of Perceived Social Support (MSPSS) and the Parental Bonding Instrument (PBI) were used as measures at baseline. Regarding absolute stability, the changes in the TAS-20 total scores and two subscales (DIF and EOT) were statistically significant but the effect sizes for the changes were small (Cohen׳s d 0.21–0.24). The test–retest correlations for the TAS-20 total and subscale scores were high (ρ=0.50–0.64, P<0.001), indicating relative stability. While several parental and social support variables were associated with alexithymia at baseline, low social support from friends was the only to predict higher alexithymia at follow-up. Alexithymia is a stable personality trait also in late adolescence. Low social support from friends is related to alexithymia in young adulthood.