Psoriasis is a heterogeneous disease for which nine linkage loci (PSORS loci 1-5 and PSORS7-10) have been accepted by the Human Genome Nomenclature Committee and an additional 16 potential susceptibility loci have been reported so far. Our previous genome-wide scan in 61 Chinese Han psoriasis vulgaris families found two susceptibility loci at 6p21.3 and 4q31 and additional suggestive linkage evidence at other regions, including 9q33. In this follow-up study, the linkage evidence at 9q33 was further investigated using an expanded sample of 160 families and improved marker coverage. Our follow-up linkage analysis of the 160 families demonstrated strong linkage evidence (P < or = 0.000022) throughout a region between 133.38 and 146.23 cM with a maximum nonparametric linkage (NPL) score of 4.64 (P = 0.00000023) and a heterogeneity LOD (HLOD) score of 5.03 (alpha = 46%) at 142.39 cM near the marker D9S290. By stratifying the 160 families into the subtypes of 130 early-onset and 30 late-onset families, we revealed stronger linkage evidence in the early-onset psoriasis families with a maximum multipoint HLOD score of 6.48 (alpha = 58%) and a maximum NPL score of 4.69 (P = 0.00000012) near marker D9S290. Our follow-up study has confirmed a novel susceptibility locus at 9q33-34 for early-onset psoriasis in the Chinese population.