Our goals were to document (1) the onset of stuttering and (2) whether specific child, family, or environmental factors predict stuttering onset in children aged up to 3 years.Participants included a community-ascertained cohort of 1619 2-year-old Australian children recruited at 8 months of age to study the longitudinal development of early language. The main outcome measure was parental telephone report of stuttering onset, verified by face-to-face expert diagnosis. Preonset continuous measures of the child's temperament (approach/withdrawal) and language development were available. Information on a range of predictor measures hypothesized to be associated with stuttering onset was obtained (maternal mental health and education levels, gender, premature birth status, birth weight, birth order, twinning, socioeconomic status, family history of stuttering).By 3 years of age, the cumulative incidence of stuttering onset was 8.5%. Onset often occurred suddenly over 1 to 3 days (49.6%) and involved the use of word combinations (97.1%). Children who stuttered were not more shy or withdrawn. Male gender, twin birth status, higher vocabulary scores at 2 years of age, and high maternal education were associated with stuttering onset. The multivariable model, however, had low predictive strength; just 3.7% of the total variation in stuttering onset was accounted for.The cumulative incidence of stuttering onset was much higher than reported previously. The hypothesized risk factors for stuttering onset together explained little of the variation in stuttering onset up to 3 years of age. Early onset was not associated with language delay, social and environmental factors, or preonset shyness/withdrawal. Health professionals can reassure parents that onset is not unusual up to 3 years of age and seems to be associated with rapid growth in language development.