Background: Intelligibility is a speaker's ability to convey a message to a listener. Including an assessment of intelligibility is essential in both research and clinical work relating to individuals with communication disorders due to speech impairment. Assessment of the intelligibility of spontaneous speech can be used as an overall indicator of the severity of a speech disorder. There is a lack of methods for measuring intelligibility on the basis of spontaneous speech. Aims: To investigate the validity and reliability of a method where listeners transcribe understandable words and an intelligibility score is calculated on the basis of the percentage of syllables perceived as understood. Methods & Procedures: Spontaneous speech from ten children with speech-sound disorders (mean age = 6.0 years) and ten children with typical speech and language development (mean age = 5.9 years) was recorded and presented to 20 listeners. Results were compared between the two groups and correlation with percentage of consonants correct (PCC) was examined. Outcomes & Results: The intelligibility scores obtained correlated with PCC in single words and differed significantly between the two groups, indicating high validity. Inter-judge reliability, analysed using intra-class correlation (ICC), was excellent in terms of the average measure for several listeners. Conclusions & Implications: The results suggest that this method can be recommended for assessing intelligibility, especially if the mean across several listeners is used. It could also be used in clinical settings when evaluating intelligibility over time, provided that the same listener makes the assessments.