Mothers of children randomly allocated to an experimental group attended fortnightly group parental language training sessions, over a 6-month period. Mothers of children allocated to a matched no intervention control group received no special attention. The results showed significantly greater gains in the expressive language skills of the experimental group compared to the control group. A second experiment was designed to compare the parental involvement approach with direct, individual treatment and to clarify the role of non-specific 'Hawthorne-type' effects. The experimental group mothers attended parental language training sessions, as above. The parental control group mothers also attended training sessions, with the emphasis on general learning skills rather than language. A third group of children received individual, direct speech and language therapy. Results showed significantly greater language gains in the parental language training group and in the individual group in comparison with the non-specific training group. The two former groups did not differ significantly, indicating that, for these groups and this methodology, parental language training is as effective as individual speech and language therapy. The results also indicate that the effectiveness of the parental involvement approach cannot be accounted for by non-specific factors. The research findings are discussed, together with the professional implications of the study and recommendations for further research.