Objective. To assess the early signs, risk factors, and rate of transition from primary Raynaud's phenomenon (primary RP) to secondary RP.Methods. A clinical sample of 307 consecutive patients with RP was included in a prospective followup study. After an initial screening, 244 patients were classified as having primary RP, of whom 236 were followed up for a mean ؎ SD of 11.2 ؎ 3.9 years. Patients classified according to the screening as having suspected secondary RP underwent an extended screening program annually until transition to secondary RP occurred.Results. The initial prevalence of secondary RP was 11%. The annual incidence of transition to suspected secondary RP was 2%, and the annual incidence of transition to secondary RP was 1%. Overall, 46 patients were classified as having suspected secondary RP, and 23 of these later were classified as having secondary RP. Older age at onset of RP (hazard ratio 2.59, 95% confidence interval [95% CI] 1.40-4.80), shorter duration of RP at enrollment (hazard ratio 0.87, 95% CI 0.81-0.94), and abnormal findings on thoracic outlet test (hazard ratio 2.69, 95% CI 1.12-6.48) were associated with an increased risk for transition to secondary RP. Compared with patients with suspected secondary RP, those diagnosed as having secondary RP had a higher number and earlier occurrence of pathologic findings. Furthermore, antinuclear antibodies at a titer of >1:320 and positive findings in specific serologic subsets were associated with a significantly increased risk for developing a connective tissue disease.