Aim: The epidemiology of preschool speech sound disorder is poorly understood. Our aims were to determine: the prevalence of idiopathic speech sound disorder; the comorbidity of speech sound disorder with language and pre‐literacy difficulties; and the factors contributing to speech outcome at 4 years.

Method: One thousand four hundred and ninety‐four participants from an Australian longitudinal cohort completed speech, language, and pre‐literacy assessments at 4 years. Prevalence of speech sound disorder (SSD) was defined by standard score performance of ≤79 on a speech assessment. Logistic regression examined predictors of SSD within four domains: child and family; parent‐reported speech; cognitive‐linguistic; and parent‐reported motor skills.

Results: At 4 years the prevalence of speech disorder in an Australian cohort was 3.4%. Comorbidity with SSD was 40.8% for language disorder and 20.8% for poor pre‐literacy skills. Sex, maternal vocabulary, socio‐economic status, and family history of speech and language difficulties predicted SSD, as did 2‐year speech, language, and motor skills. Together these variables provided good discrimination of SSD (area under the curve=0.78).

Interpretation: This is the first epidemiological study to demonstrate prevalence of SSD at 4 years of age that was consistent with previous clinical studies. Early detection of SSD at 4 years should focus on family variables and speech, language, and motor skills measured at 2 years.