The fine detection of 11-nor-Δ9-tetrahydrocannabinol-9-carboxylic acid (THCCOOH) in hair matrix remains one of the most important topics in hair analysis. This relevance lies in the necessity to obtain evidence of effective drug consumption and dispel any doubt of environmental contamination. THCCOOH is the highest and mainly represented Δ9-THC metabolite, but its concentration in hair is very low. A sensitive method for quantitative determination of THCCOOH in hair was developed. As first step, the method was tested with different SPE/LLE conditions, but the best results were obtained with a simple ad hoc LLE extraction. The final method was fully validated, evaluating parameters like extraction recovery, linearity, specificity and sensitivity. More than one hundred hair samples were then analyzed with the validated method. Data analysis was performed so as to determine respective concentrations of the metabolite and active molecule. Hair was washed and cut into small pieces (2-4mm). Samples (20-50mg) were spiked with deuterated internal standard (THC-d(3) and THCCOOH-d(3)) and then hydrolyzed at 90°C in 1mL of 1M NaOH for 15min. THC was isolated by a LLE basic extraction with n-hexane:ethyl acetate (9:1). Next the aqueous solution was acidified (pH 4) adding concentrated acetic acid. THCCOOH was extracted with the same mixture. Dried extracts were derivatized with pentafluoropropionic anhydride and hexafluoroisopropanol and analyzed by GC/MS/MS (Agilent 7000B triple quadrupole) in NCI mode. The linear range of THCCOOH is 0.1-5pg/mg, with good correlation coefficients (r(2)>0.9993). This method has great sensitivity (LOD 0.01pg/mg to LOQ 0.04pg/mg), high recovery, reproducibility and robustness. Based on these results, the method proved to be effective for the rapid determination of THC and THCCOOH in hair specimens.