In the course of a 2.2 micron monitoring programme of nearby Seyfert 1 galaxies, we have detected an outburst in NGC 4051 in which the source flared and dimmed by more than a factor of 2 in 6 months. The UV flux, although sparsely sampled, is less variable than the near infrared. The available data suggest that dust reprocessing of the UV radiation from the nucleus is an acceptable explanation for the variable NIR component only if the light travel time to the dust is shorter than the UV variability timescale, and the NIR to UV ratio remains nearly constant. Alternatively, the NIR could be direct non-thermal emission from the nucleus. In either case NGC 4051 does not fit into the same scheme as Fairall 9 and NGC 3783, where the UV variations are the dominant ones, and are reprocessed into NIR radiation after a non- negligible delay. A supernova explosion in a heavily obscured starburst is excluded.