We present the Need of Support and Service Questionnaire (NSSQ), a new instrument developed to provide professionals in psychiatric care and community social services with a tool to assess needs of support and service in mentally disabled clients. The final version of the instrument was designed after comments from professionals in psychiatric care, community social services, and the clients and their relatives. A reliability study (test-retest, n=77), inter-rater (n=69) and a validity study (n=529) were performed. In the validity study, comparisons were done with the Camberwell Assessment of Need (CAN) scale, the Global Assessment of Functioning (GAF) scale and the Social and Occupational Functioning Assessment Scale (SOFAS). The test-retest and inter-rater reliability were 0.86 and 0.76, respectively. The percentage of agreement on the individual items ranged from 83.1% to 100.0% (test-retest) and 76.8% to 100.0% (inter-rater reliability). The corresponding kappa coefficients ranged from 0.63 to 1.00 (test-retest) and 0.33 to 1.00 (inter-rater reliability). The comparison between NSSQ and CAN items demonstrated high sensitivity and specificity as well as satisfactory likelihood ratios (LRs). The correlations between GAF and SOFAS scores and the number of needs of support, number of needs of service and the total number of needs of support and service were -0.49, -0.32 and -0.47, and -0.48, -0.26 and -0.45, respectively. All correlations were statistically significant (P<0.01). The results demonstrated that the psychometric characteristics of the NSSQ were satisfactory. Moreover, professionals without any formal training easily used it.