Background. Primary hyperoxaluria type 1 (PH1) is a phenotypically heterogeneous disease. To date the relationship between biochemical parameters and outcome is unclear. We therefore undertook a national cohort study on biochemical and clinical parameters and outcome in PH1. Methods. Review of medical charts of all Dutch PH1 patients, who were identified by sending questionnaires to all Dutch nephrologists for children and adults. Results. Fifty‐seven patients were identified. The prevalence and incidence rates were 2.9/106 and 0.15/106/year, respectively. Median age at diagnosis was 7.3 years (range 0–57). Seventeen (30%) patients were older than 18 years at time of diagnosis, of whom 10 (59%) presented with end‐stage renal disease (ESRD), in contrast to only nine (23%) of those aged under 18 years. Median age at initial symptoms was 6.0 years (range 0–50). In four of nine patients with infantile PH1, normal renal function was preserved after a median follow‐up of 7.7 years (range 0.1–16). Progression to renal insufficiency was associated with the presence of nephrocalcinosis, as assessed by ultrasound (relative risk=1.8; 95% CI, 1.0–3.4) and with pyridoxine‐unresponsiveness (relative risk=2.2; 95% CI, 1.1–4.2) but not with age at presentation, the extent of hyperoxaluria, or AGT activity. No apparent nephrocalcinosis was found in five of the 19 patients who presented with ESRD. Conclusions. Although more than one‐half of the PH1 patients have symptoms under the age of 10 years, PH1 can present at any age. In adults, PH1 presents predominantly with ESRD, which may be due to misinterpretation of early symptoms. Although nephrocalcinosis is correlated with development of renal insufficiency, the latter can occur even in the absence of nephrocalcinosis. Pyridoxine sensitivity is associated with better outcome in PH1.