Seven hypervariable VNTR loci have been studied in about 1200 parent/child pairs about equally divided between the sexes. Mutations were observed with all seven probes, the total number being 71. Fourty-four of these involved increased fragment length. Gains in fragment length were on average larger than losses. These findings indicate that mutation might be a basis for evolutionary expansion of VNTR fragment length. For five probes YNH24 (D2S44), MS31 (D7S21), g3 (D7S22), MS43A (D12S11), and CMM101 (D14S13), mutation rates were relatively low (less than 1%) with no obvious sex difference. MS1 (D1S7) mutation frequencies were substantially higher, with a tendency towards a higher paternal than maternal mutation rate (5.4% and 2.0%, respectively). The probe B6.7 (provisionally assigned to chromosome 20) exhibits about five times higher paternal than maternal mutation rates. The mutation rate of 7.6% in paternal chromosomes is among the highest reported in any VNTR locus. These findings could indicate that while low-mutant VNTRs might reflect meiotic crossover, mutation events in high-mutant loci could more often be caused by other mechanisms during cell division.