A note on basic Iwasawa λ-invariants of imaginary quadratic fields and congruence of modular forms by Dongho Byeon (Seoul)1. Introduction and statement of results. For a number field k and a prime number l, we denote by h(k) the class number of k and by λ l (k) the Iwasawa λ-invariant of the cyclotomic Z l -extension of k, where Z l is the ring of l-adic integers.Let l be an odd prime number. Using the Kronecker class number relation for quadratic forms, Hartung [3] proved that there exist infinitely many imaginary quadratic fields k whose class numbers are not divisible by l. For the case l = 2, this is an immediate consequence of Gauss' genus theory. On the other hand, using the idea of Hartung and Eichler's trace formula combined with the l-adic Galois representation attached to the Jacobian variety J = J 0 (l) of the modular curve X = X 0 (l), Horie [4] proved that there exist infinitely many imaginary quadratic fields k such that l does not split in k and l does not divide h(k). Later Horie and Onishi [5] obtained more refined results. By a theorem of Iwasawa [6], these results imply that there exist infinitely many imaginary quadratic fields k with λ l (k) = 0. For the case l = 2, this is also an immediate consequence of Gauss' genus theory. For the case l = 3, by refining Davenport and Heilbronn's result [2], Nakagawa and Horie [8] gave a positive lower bound on the density of imaginary quadratic fields k and real quadratic fields k with λ l (k) = 0.