Key words Sturm-Liouville problem, Prüfer angle, asymptotics of eigenvalues MSC (2000) 34L20To the memory of a friend who still teaches us so much Atkinson's semi-definite Sturm-Liouville problem consists of the differential equation −(y /s) + qy = λry, with s, q, r integrable on [a, b], q real-valued, s, r ≥ 0, and separated boundary conditions at a, b. The asymptotic behavior of the associated Prüfer angle is determined as λ → ±∞. This leads to existence theorems for eigenvalues λn with prescribed oscillation number n and their asymptotics. In general, λn grows faster than n 2 , and the order of the corresponding characteristic function is less than 1 2 .