Psoriasis is clinically a heterogeneous disease. Detailed evaluation of phenotype at disease onset is lacking. This study is a baseline characterization of 400 adult individuals with first time incidence of psoriasis on non-hairy skin, describing clinical phenotypes and putative environmental triggers at disease onset. In total, 74 patients with guttate and 326 patients with non-guttate phenotype, the majority with plaque psoriasis, were included. Guttate phenotype was associated with younger age and recent infection in 84%, where acute streptococcal pharyngitis was verified in 63%. The predominating factor associated with onset of plaque psoriasis was a recent life crisis (46%). A positive family history for psoriasis was approximately the same in both groups. Psoriasis arthropathy was diagnosed in 5% of guttate and 15% of non-guttate patients, with enthesopathy being the dominant symptom among guttate patients. This study confirms the strong link between onset of guttate psoriasis phenotype and streptococcal throat infection, whereas onset of plaque psoriasis was highly associated with a preceding distinct stressful life event. Longitudinal follow-up of the patients will provide robust information about disease development and response to treatment.