Aim: A little is known about predictive validity of and professionals' adherence to language screening protocols. This study assessed the concurrent and predictive validity of the Dutch well child language screening protocol for 2-year-old children and the effects of protocol deviations by professionals. Methods: A prospective cohort study of 124 children recruited and tested between October 2013 and December 2015. Children were recruited from four well child clinics in urban and rural areas. To validate the screening, we assessed children's language ability with standardised language tests following the 2-year screening and 1 year later. We assessed the concurrent and predictive validity of the screening and of protocol deviations. Results: At 2 years, the sensitivity and specificity of the language screening were 0.79 and 0.86, and at 3 years 0.82 and 0.74, respectively. Protocol deviations by professionals were rare (7%) and did not significantly affect the validity of the screening. Conclusion: The language screening protocol was valid for detecting current and later language problems. Deviations from the protocol by professionals were rare and did not affect the concurrent nor predictive validity of the protocol. The 2-year language screening supports professionals working in preventive child health care and deserves wider implementation in well child care.