The objective of this study was to develop and validate a scale to assess the diurnal impact of insomnia. The Insomnia Diurnal Impact Scale (IDIS) comprises six items designed to evaluate the daytime effects of insomnia. The sychometric properties of the original scale were analysed in a sample of 172 students, while its ability to differentiate insomniacs and non-insomniacs (according to the International Classification of Diseases (ICD)-10 criteria) was examined in a sample of 79 psychiatric patients and 82 individuals from the community. The psychometric properties of the English version were then analysed in a sample of 44 Englishspeaking participants. The results showed the internal consistency coefficient to be very good (0.86), with test retest reliability at 1 month being 0.79. A single factor explained almost 60% of the variance. Correlation of the IDIS with other scales varied between moderate and high values. Sensitivity was 78% and specificity 57% in the community sample, while the corresponding figures for the psychiatric population were 83% and 63%. Cronbach's ¿ coefficient for the English version reached a value of 0.93. These results indicate that the IDIS shows adequate reliability and validity with both general and psychiatric populations, and also that it can discriminate between the presence and absence of insomnia. The English version presents good preliminary results regarding item-corrected total correlation and internal consistency. In conclusion, the IDIS appears to be a useful tool in the primary care and mental health contexts for assessing insomnia-related diurnal dysfunction.