The percentage and absolute number per mm.3 of peripheral T-lymphocytes were determined in 11 juvenile-onset diabetics (JOD), in 21 maturity-onset diabetics (MOD), and in 18 normal subjects (NS). The percentage was significantly lower in JOD (38.1) than in MOD (57.2) and NS (56.5). The absolute T-lymphocytes number per mm.3 was significantly lower in JOD (833) than in NS (1,260); this was also true for JOD as against MOD (1,026), even if the difference was not statistically significant. No difference was found between MOD and NS, or between MOD on oral therapy and on insulin treatment. The decrease of peripheral T-lymphocytes in JOD was not related to associated illness or drugs. The data presented suggest the possibility of an altered cell-mediated immunity in juvenile-onset diabetics.