PurposeTo evaluate the influence of riociguat on World Health Organization functional class (WHO FC), 6-min walk distance (6MWD), right heart remodeling, and right ventricular–pulmonary arterial (RV–PA) coupling in patients with idiopathic pulmonary arterial hypertension (IPAH) who are treatment-naïve or who have failed to achieve treatment goals with sildenafil therapy.MethodsTwenty patients with IPAH were enrolled: 12 had not previously received PAH-targeted therapy (treatment-naïve subgroup) and 8 had been receiving sildenafil therapy but failed to achieve treatment goals; on entering this pilot study these 8 patients were switched from sildenafil to riociguat therapy (treatment-switch subgroup). Patients received riociguat individually dose-adjusted up to a maximum of 2.5 mg three times daily. After 12 weeks, patients were assessed for WHO FC, 6MWD, right heart remodeling, and RV–PA coupling.ResultsRiociguat significantly improved WHO FC in treatment-naïve patients (from 0/4/8/0 patients in WHO I/II/III/IV at baseline to 1/6/5/0 at week 12) and in treatment-switch patients (from 0/4/4/0 patients in WHO I/II/III/IV at baseline to 1/4/3/0 at week 12). Additionally, treatment-naïve and treatment-switch patients showed significant improvements at week 12 versus baseline in 6MWD (increases of + 76.8 m and + 71.6 m, respectively), RV systolic function, and RV–PA coupling.ConclusionThese results support the proven efficacy of riociguat in patients with IPAH, including treatment-naïve patients and those switching to riociguat following failure to achieve treatment goals with sildenafil, and suggest that it may be possible to delay disease progression in this patient group.Electronic supplementary materialThe online version of this article (10.1007/s00408-018-0160-4) contains supplementary material, which is available to authorized users.