Objective: to develop and validate a family functioning assessment instrument. Methods: validation study, conducted at the Family Medicine Unit No. 80 of the Mexican Institute of Social Security in Morelia, Michoacán, Mexico; from March 2018 to August 2019. Phase 1. The items were written using the McMaster model; phase 2. Test adequacy through qualitative analysis of items: presentation of the instrument, wording clarifying, relevance of variables to indicators, relevance of content and feasibility of application, application of statistical tests; item reduction, relevance, varimax rotation and variance; phase 3. Reliability and convergence validity against ff-sil. Results: phase 1. Construction of an initial instrument of 107 items with six domains. Phase 2. After a pilot test on one hundred individuals, 79 items with relevance >90 were chosen; and after a factorial analysis with a reduction factor, made by ten judges, a final instrument of 35 items was created with redistribution of the items by the varimax rotation method, six domains were assigned: problem solving (7 items), communication (10 items), roles (5 items), affective involvement (2 items), affective responses (8 items) and behavioral control (3 items); with a total variance of 74.1. Phase 3. Final instrument of 35 items that categorizes families into: functional (141-175 points), mild dysfunctional (106-140 points), moderate dysfunctional (71-105 points) and severe dysfunctional (35-70 points). Final instrument with Cronbach’s alpha of 0.962 vs. 0.905 of the ff-sil. Conclusion: a reliable 35-item instrument measuring family functioning was constructed.