Abstract Aims Unexplained sudden cardiac death (SCD) may be attributable to cardiogenetic disease. Presence or absence of autopsy anomalies detected following premature sudden death direct appropriate clinical evaluation of at-risk relatives towards inherited cardiomyopathies or primary arrhythmia syndromes, respectively. We investigated the relevance of non-diagnostic pathological abnormalities of indeterminate causality (uncertain) such as myocardial hypertrophy, fibrosis, or inflammatory infiltrates to SCD. Methods and results At-risk relatives of unexplained SCD cases aged 1–64 years without prior cardiac disease (n = 98) with either normal and negative (40%, true sudden arrhythmic death syndrome; SADS) or isolated non-diagnostic (60%, uncertain sudden unexplained death; SUD) cardiac histological autopsy findings at a central forensic pathology unit were referred to the regional unexplained SCD clinic for clinical cardiac phenotyping. Uncertain SUD were older than true SADS cases (31.8 years vs. 21.1 years, P < 0.001). A cardiogenetic diagnosis was established in 24 families (24.5%) following investigation of 346 referred relatives. The proportions of uncertain SUD and true SADS explained by familial cardiogenetic diagnoses were similar (20% vs. 31%, P = 0.34, respectively), with primary arrhythmia syndromes predominating. Unexplained SCD cases were more likely than matched non-cardiac premature death controls to demonstrate at least one uncertain autopsy finding (P < 0.001). Conclusion Primary arrhythmia syndromes predominate as familial cardiogenetic diagnoses amongst both uncertain SUD and true SADS cases. Non-diagnostic or uncertain histological findings associate with SUD, though cannot be attributed a causative status. At-risk relatives of uncertain SUD cases should be evaluated for phenotypic evidence of both ion channel disorders and cardiomyopathies.